Elite Iraqi troops battled the Islamic State group in the streets of Mosul Friday, as the UN reported jihadists had executed dozens of people inside the city for alleged "treason".

With IS also on the defensive in neighbouring Syria, US-backed forces pressed an advance on jihadist bastion Raqa after a sandstorm eased.

High winds in the desert separating the Syrian Kurdish-Arab militia alliance from the jihadists' Euphrates Valley stronghold had slowed their advance Thursday as visibility plummeted.

Iraqi forces had also regrouped after meeting stronger than expected resistance from IS on the east bank of the Tigris River running through Mosul after thrusting into the built-up area last week.

Commanders of Iraq's elite Counter-Terrorism Service (CTS) said troops were advancing on two eastern neighbourhoods of the city.

In a house near the front line, Staff Lieutenant Colonel Muntadhar Salem clutched a radio in one hand and a tablet computer in the other with a map showing recaptured buildings.

As the troops waited for orders to push forward, incoming mortar rounds shook curtains on the windows.

Later Friday, the fighting eased off, although CTS forces still occasionally fired mortar rounds towards IS positions in eastern Mosul's Arbajiyah area.

IS fighters reportedly shot dead more than 60 people this week and hung some of the bodies from poles inside Mosul after claiming they had collaborated with Iraqi troops, the UN human rights office said Friday.

- 'Treason, collaboration' -

"On Tuesday, ISIL (IS) reportedly shot and killed 40 civilians in Mosul city after accusing them of 'treason and collaboration'", it said.

On Wednesday, IS slaughtered another 20 people at the Ghabat Military Base in northern Mosul after accusing them of "leaking information", it added.

The battle for Mosul is now in its fourth week, and while troops have entered the built-up area, there are weeks, if not months, of fighting ahead.

"Our forces have begun the attack on Arbajiyah. The clashes are ongoing," Salem said.

Another CTS officer, Staff Lieutenant Colonel Ali Hussein Fadhel, said the first row of buildings in Arbajiyah had been seized.

"We are within firing range of Karkukli but the full attack has not yet started," he said of another eastern district.

Iraq began the operation to retake Mosul on October 17, with federal and Kurdish regional forces closing in on the city from three sides.

Pro-government Shiite paramilitaries later began advancing on the town of Tal Afar, which commands the city's western approaches, with the goal of cutting the jihadists off from territory they control in Syria.

The advance up the Tigris Valley from the south has been slowest. Troops on that front had the farthest to cover and a string of jihadist-held towns in their path.

On Thursday, the battle neared the remains of ancient Nimrud, some 30 kilometres (20 miles) south of Mosul, raising fears for the famed heritage site already ravaged by jihadist explosives and sledgehammers.

- SDF gains -

The International Organization of Migration said Friday more than 47,000 people have been displaced since the Mosul operation began.

In Syria, the US-backed Syrian Democratic Forces (SDF) said their advance on Raqa was back on track after a sandstorm swept through the area on Thursday.

"We seized control of two new villages yesterday but we didn't advance as far as planned because of the sandstorm," SDF commander Merkhas Kamishlo said.

Fighting had focused on IS-held Al-Heisha, some 40 kilometres north of Raqa, and the SDF reported late Friday its forces took control of that village and three others on the Suluk front.

Early Friday, an AFP correspondent reported heavy air strikes by the US-led coalition supporting the advance.

"Al-Heisha is totally besieged and overnight the US-led coalition hit four Daesh positions inside the village, and destroyed a vehicle being prepared for use as a bomb," Kamishlo said, using an Arabic acronym for IS.

The SDF offensive, launched last weekend, has been pushing south from areas near the Turkish border.

The alliance said late Friday its forces advancing south from the towns of Ain Issa and Suluk had now converged.

The military in Russia, which has sided with the Damascus regime, said Friday it had evidence rebels in Aleppo city used chemical weapons, a charge the opposition denied.

The global chemical arms watchdog, meanwhile, condemned both Syria and IS for using toxic weapons and called for stepped up inspections.

A joint panel of the United Nations and the Organization for the Prohibition of Chemical Weapons found regime forces carried out three chlorine gas attacks in 2014 and 2015, while IS used mustard gas in Syria in 2015.