US-backed Kurdish-Arab forces launched an offensive Sunday on the Islamic State group's de facto Syrian capital Raqa, upping pressure on the jihadists who are already battling Iraqi troops in Mosul.

The start of the assault by the Syrian Democratic Forces (SDF) came as Iraqi forces fought inside Mosul for the third day running amid fierce jihadist resistance.

The two cities are the last major urban centres under IS control after the jihadists suffered a string of territorial losses in Iraq and Syria over the past year.

The US-led coalition battling IS is backing both assaults, hoping to deal a knockout blow to the self-styled "caliphate" it declared in mid-2014.

SDF commanders announced the start of the Raqa operation in Ain Issa, some 50 kilometres (30 miles) north of the city.

"The major battle to liberate Raqa and its surroundings has begun," SDF spokeswoman Jihan Sheikh Ahmed said.

Operation "Wrath of the Euphrates" involves some 30,000 fighters and began on Saturday night, Ahmed said.

SDF forces are advancing on three fronts, from Ain Issa and Tal Abyad to the north of Raqa, and from the village of Makman to the east.

SDF spokesman Talal Sello told AFP forces would first seize areas around Raqa before taking the city itself.

"The fight will not be easy, and will require accurate and careful operations because IS will defend its bastion knowing that the loss of Raqa will mean it is finished in Syria," Sello said.

- SDF gains -

US Defence Secretary Ashton Carter echoed that sentiment.

"As in Mosul, the fight will not be easy and there is hard work ahead, but it is necessary to end the fiction of ISIL's caliphate and disrupt the group's ability to carry out terror attacks against the United States, our allies and our partners," Carter said, using an alternative name for IS.

An AFP correspondent in Ain Issa Sunday saw dozens of SDF fighters heading for the front line.

SDF spokeswoman Ahmed said that 10 villages and several hamlets had been retaken.

Later the powerful Kurdish People's Protection Units (YPG) later denied an IS report that 14 of its fighters were killed in a car bomb attack the Suluk area.

The Syrian Observatory for Human Rights monitor reported only wounded in that attack.

Driving IS from Mosul and Raqa has been the endgame since the US-led coalition launched air strikes against it in summer 2014.

The coalition has also provided training and deployed hundreds of advisers to work with Iraqi forces and select Syrian fighters, including the SDF.

Near Ain Issa, the AFP correspondent saw at least one soldier who had US markings on his helmet with SDF fighters.

Sello said the alliance had received new weapons from the coalition for the Raqa battle, including anti-tank missiles.

Another SDF source said 50 US military advisers would be involved in the operation, particularly to guide air strikes.

- Jihadist atrocities -

After it was seized by IS, Raqa saw some of the jihadists' worst atrocities, from stonings and beheadings to the trading of sex slaves.

Last month, the US defence secretary said the idea of simultaneous operations against Mosul and Raqa "has been part of our planning for quite a while".

But the battle for Raqa is far more complicated.

After five years of civil war, Syria is divided into a patchwork of fiefdoms, with President Bashar al-Assad's regime, IS and a range of opposition forces all holding territory.

Dominated by the People's Protection Units, the SDF has in recent months flushed IS out of swathes of territory in northern Syria, including the flashpoint town of Manbij in August.

Washington has promoted the SDF as a key ally in the fight against IS, but the partnership is complicated by Turkey's fierce opposition to the YPG.

Ankara considers the militia a "terrorist" group, and in August began its own operation inside northern Syria, targeting both IS and the YPG.

Sello said the SDF had agreed with Washington "that there will be no role for Turkey or the armed factions allied with it in the operation" to capture Raqa.

In Jordan, however, President Barack Obama's envoy Brett McGurk said Washington was in "close contact" with Ankara over the assault.

- 'Complex environment' -

"It is a complex environment in Syria to say the least, but we are constantly in touch with all the different players."

In Mosul, Iraqi forces were clearing eastern neighbourhoods, nearly three weeks into the offensive there.

"Resistance is very heavy and they (IS) have suffered major losses," Staff Lieutenant General Abdelghani al-Assadi of the elite Counter-Terrorism Service told AFP.

Soldiers from the army's 9th Armoured Division also battled jihadists in a southeastern neighbourhood, an AFP correspondent reported.

IS has responded to the Mosul assault with a string of diversionary attacks.

It claimed responsibility for suicide bombings on Sunday in Tikrit and Samarra, two cities north of Baghdad. Officials said at least 25 people were killed and more than 50 wounded.

Aid groups have voiced concerns for civilians trapped in both Mosul and Raqa, warning they may be used as human shields.

More than a million people are believed to be in Mosul. Raqa had a population of some 240,000 before 2011 but more than 80,000 people have since fled there from other parts of Syria.

Key events in the war against IS
Beirut (AFP) Nov 6, 2016 - A Kurdish-Arab alliance announced Sunday an assault against the Islamic State group's Syrian bastion of Raqa.

In neighbouring Iraq, government forces are also battling to oust the group from Mosul, the country's last IS-held city.

Here are milestones in the fight against the jihadist group.

First international air strikes

On August 8, 2014, US warplanes strike IS positions in northern Iraq in response to an appeal from Baghdad.

Already entrenched in neighbouring Syria, IS seizes swathes of Iraqi territory in a lightning offensive launched in June 2014 that sees Iraq forces collapse and flee.

On September 5, US President Barack Obama vows to build an international coalition to defeat the group.

On September 23, the US and Arab allies launch air strikes on IS in Syria.

IS driven out of Kobane

IS is driven out of the Syrian border town of Kobane on January 26, 2015 after more than four months of fighting led by Kurdish forces backed by coalition air strikes.

An Iraqi military official also says the eastern province of Diyala has been liberated.

Tikrit recaptured, Ramadi lost

On March 31, 2015, Iraq announces the "liberation" of Tikrit, 160 kilometres (100 miles) north of Baghdad, though fighting in the city continues for days afterwards.

Government forces and Shiite militias had begun their offensive on March 2 to oust the group, which had controlled Tikrit for nearly 10 months.

But in May, IS overruns Ramadi, capital of the Iraqi province of Anbar, and Syria's UNESCO-listed ancient city of Palmyra.

IS ousted from border town

On June 16, 2015, Kurdish militia backed by Syrian rebels and coalition air strikes seize the town of Tal Abyad on the Syrian border with Turkey from IS, which had occupied it for more than a year.

Tal Abyad was one of two main transit points on a key supply route to Raqa.

Turkey declares war on IS

On July 24, 2015, Turkish warplanes bomb IS positions inside Syria for the first time, in a dramatic toughening of Ankara's stance.

Turkish raids are however mostly aimed at positions of the Kurdistan Workers' Party (PKK) in Iraq and Syria.

Turkey gives Washington the go-ahead to launch strikes against IS in Syria from Ankara's southern air base of Incirlik.

Russia intervenes in Syria

On September 30, 2015, Russia launches air strikes in Syria to help its Syrian regime ally. Washington accuses Moscow of targeting US-backed Syrian rebels rather than IS positions.

Iraq's Sinjar, Ramadi retaken

On November 13, 2015, Iraqi Kurds announce the recapture of the northern Sinjar region from IS.

In Syria, a coalition of Arab and Kurdish fighters drives IS out of the village of Al-Hol. The US-led coalition provides air support.

Iraqi forces announce the recapture of Ramadi in December, but security forces do not establish full control over the area until February.

Syrians retake Palmyra

On March 27, 2016, Russian-backed Syrian forces retake Palmyra.

Fallujah recaptured

Iraqi forces recapture Fallujah in June 2016 after two and a half years in which the city was outside government control.

Anti-government fighters had seized Fallujah at the beginning of 2014 and it became an IS stronghold.

With Fallujah's recapture, second city Mosul becomes the last major IS urban stronghold in Iraq.

Manbij falls

On August 6, the Syrian Democratic Forces coalition of Arab and Kurdish fighters backed by US air strikes recaptures the northern Syrian town of Manbij following a two-month battle against IS.

Jihadists lose border area

On September 4, Turkish troops and allied rebel fighters drive IS from its last positions along the Turkish-Syrian border.

IS loses Dabiq

Syrian rebels deal a major symbolic blow to IS in mid-October, recapturing the Syrian town of Dabiq, where the jihadists had promised an apocalyptic battle.

The battle for Mosul

A coalition of Iraqi federal and Kurdish forces backed by US-led air support launches an operation to retake Mosul on October 17, closing in from the north, east and south.

Iraqi pro-government paramilitary forces announce a drive aimed at opening a western front on October 29. They retake a series of villages as they close in on Tal Afar town between Mosul and the Syrian border.

Iraqi forces reach Mosul's eastern outskirts some two weeks into the battle, and are now fighting inside the city, where they have faced fierce resistance.

Forces on the southern front, which had the longest way to go, are still about 14 kilometres from Mosul's outskirts, while those on the northern side have advanced closer to the city.