US-backed Kurdish-Arab forces launched an offensive 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) 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.

- Diversionary tactics -

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.

On Monday, Amnesty International said that Iraqi Kurdish forces have driven out hundreds of Arabs from Kirkuk, in northern Iraq, and destroyed their homes, apparently in retaliation for an attack by IS on October 21.

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.

Source: AFP