The United States and its Arab allies unleashed deadly bomb and missile strikes on jihadists in Syria on Tuesday, opening a new front in the battle against the Islamic State group.

Dozens of IS and Al-Qaeda militants were reported killed in the raids, which Washington said had partly targeted extremists plotting an "imminent attack" against the West.

Bahrain, Jordan, Qatar, Saudi Arabia and the United Arab Emirates joined the operation, which involved fighter jets, bombers, drones and Tomahawk missiles fired from US warships.

President Barack Obama, who had already launched strikes against IS militants in neighbouring Iraq, said "the strength of this coalition makes it clear to the world that this is not America's fight alone.

"It must be clear to anyone who would plot against America and try to do Americans harm that we will not tolerate safe havens for terrorists who threaten our people," he added.

The Pentagon described the strikes as "very successful".

It said they hit targets including IS positions, training compounds, command centres and armed vehicles in the jihadist stronghold of Raqa and near the border with Iraq.

The raids marked a turning point in the war against IS, which has seized swathes of Syria and Iraq and declared an Islamic "caliphate".

The fact that the five Arab nations joining the strikes are Sunni-ruled will also be of crucial symbolic importance in the fight against the Sunni extremists of IS.

- Turkey to 'join coalition' -

NATO member Turkey, another Sunni nation and a neighbour of Syria, has so far remained on the sidelines, but US Secretary of State John Kerry said Ankara had pledged to join the coalition.

"Turkey is very much part of this coalition, and Turkey will be very engaged on the frontlines of this effort," Kerry said after meeting Turkish officials in New York.

Speaking on national television, Turkish President Recep Tayyip Erdogan said Ankara's contribution would consist of "all kinds of support including military and political".

Washington had been reluctant to intervene in Syria's civil war, but was jolted into action as the jihadists captured more territory and committed atrocities including the beheadings of three Western hostages.

The United States said it had not requested Syria's permission for the strikes or given advance notice "at a military level".

Even so, President Bashar al-Assad said Damascus supported any international effort "to fight against terrorism".

The United Nations says about 191,000 people have been killed since an uprising against Assad erupted in 2011, escalating into a brutal civil war that brought jihadists streaming into Syria.

- 'Huge impact' -

An anti-regime activist in Raqa, Abu Yusef, said IS had redeployed its fighters in response.

"The impact of the strikes has been huge," and the jihadists "are focused on trying to save themselves now," he told AFP by Internet.

The raids prompted many civilians living near IS positions across Syria to flee, according to the Syrian Observatory for Human Rights, a Britain-based monitoring group.

That follows the exodus in recent days of tens of thousands of residents into neighbouring Turkey in response to an IS assault on a strategic Kurdish town in northern Syria.

IS militants have warned of retaliation to the strikes, and Algerian extremists threatened Monday to kill a French hostage within 24 hours unless Paris halted air raids in Iraq.

French Prime Minister Manuel Valls ruled out negotiation and rejected their demand.

- 'Al-Qaeda plot' -

Washington said it launched 14 strikes against IS targets around the jihadist stronghold of Raqa, as well as in Deir Ezzor, Albu Kamal and Hasakeh on the border with Iraq.

The raids were believed to have killed a group of Al-Qaeda "veterans" -- known as Khorasan -- plotting to attack the United States and Western interests, the Pentagon said.

The strikes targeted their training camps, an explosives and munitions production site, a communication building and a command and control centre, it said.

The Syrian Observatory said at least 50 Al-Qaeda militants were killed, as well as more than 70 members of IS. Eight civilians, including three children, were also among the dead.

The CIA has said IS has up to 31,000 fighters, and EU counter-terrorism chief Gilles de Kerchove told AFP Tuesday the number of Europeans joining Islamist fighters in Syria and Iraq has surged to about 3,000 from 2,000 only months ago.

Air strikes were also conducted in Iraq, the Pentagon said, bringing the total number of US raids there to 194.

Washington has said the goal of the strikes is to weaken IS so it can be taken on by ground forces including the Iraqi army and moderate Syrian rebels, who are to be trained and equipped by the coalition.

Syria's opposition -- which had pleaded for the strikes -- welcomed the new raids, but urged sustained pressure on Assad's government.

In other developments, a car bomb exploded near a market in Baghdad's Shiite neighbourhood of Sadr City, killing at least 14 people, police and medical sources said.

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