US-backed Kurdish-Arab forces launched an offensive Sunday on the Islamic State group's de facto Syrian capital Raqqa, 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, with the jihadists putting up fierce resistance.

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

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

Lined up in crisp fatigues at an outdoor press conference, SDF commanders announced the start of the operation against Raqqa in Ain Issa, some 30 miles north of the city.

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

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

SDF spokesman Talal Sello told AFP it would proceed in two phases, first seizing areas around Raqqa and isolating the city, then taking control of the city itself.

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

'Fight will not be easy'

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

An AFP correspondent in Ain Issa saw dozens of SDF fighters heading on vehicles towards the front line.

Driving the jihadists from Mosul and Raqqa has been the endgame since the US-led coalition launched air strikes against ISIS in the summer of 2014.

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

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

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

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

Thousands of foreign fighters flocked there to join IS, and US officials have described it as the nerve centre for the group's attacks abroad.

In Washington, a US official confirmed the start of the operation to capture the stronghold.

"We will first undertake an effort to isolate Raqqa to set the stage for an eventual assault on the city itself to liberate it," the official said, speaking on condition of anonymity.

Tensions with Turkey

Last month, US Defense Secretary Ashton Carter said the idea of simultaneous operations against Mosul and Raqqa "has been part of our planning for quite a while".

But the battle for Raqqa 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, ISIS and a range of opposition forces all holding territory.

Dominated by the powerful Kurdish People's Protection Units (YPG), the SDF has in recent months flushed ISIS 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 ISIS, 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 on Sunday that the SDF had "agreed definitively" with the United States "that there will be no role for Turkey or the armed factions allied with it in the operation" to capture Raqqa.

General Joseph Dunford, the chairman of the US joint chiefs of staff, on Sunday made a previously unannounced visit to Ankara for talks with his Turkish counterpart, but no further details were immediately available.

'Heavy resistance' in Mosul

In Mosul, Iraqi forces were clearing eastern neighborhoods of the city on Sunday, nearly three weeks into the offensive to retake the city.

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

Soldiers from the army's 9th Armored Division also battled jihadists in a southeastern neighborhood of Intisar, an AFP correspondent reported.

ISIS has responded to the Mosul assault with a string of diversionary attacks elsewhere in Iraq, including spectacular operations in Kirkuk and Rutba.

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

Aid groups have raised deep concerns for civilians trapped in both Mosul and Raqqa, with warnings the jihadists are likely to use them as human shields.

More than a million people are believed to be in the Iraqi city. Raqqa had a population of some 240,000 before the start of Syria's civil war and more than 80,000 people have since fled there from other parts of the country.

By AFP's Delil Souleiman in Ain Issa, Syria, with Sarah Benhaida in Mosul, Iraq.