QAMISHLI, Syria (AFP) - US-backed fighters fought fierce battles Thursday as they tried to push further into the Islamic State group's Syrian bastion Raqa, two days after finally entering the northern city.

The Syrian Democratic Forces alliance of Kurdish and Arab fighters began the battle for the city earlier this week after seven months of fighting to surround the jihadist stronghold.

On Wednesday, an AFP journalist entered the city with SDF fighters and witnessed heavy clashes in the Al-Meshleb neighbourhood, with IS firing multiple mortar rounds towards the advancing forces.

He said part of the neighbourhood was under SDF control but US-led coalition planes were still carrying out strikes against IS fighters elsewhere in the district, one of the largest in Raqa.

SDF fighters were armed mostly with light weapons including Kalashnikovs, and were also returning mortar fire on IS positions.

They were trying to conceal their vehicles from attack by armed drones dispatched by IS fighters, a favoured technique of the jihadist group in its battles in Syria and Iraq.

The SDF did not allow journalists to return to the city on Thursday but the Syrian Observatory for Human Rights said fierce fighting was continuing in Al-Meshleb.

The Britain-based monitoring group said the SDF now controlled around two-thirds of the district and was some 400 metres (yards) from the neighbouring Al-Senaa district.

"IS has snipers monitoring Al-Meshleb neighbourhood and has laid mines extensively throughout it," the Observatory said.

The group said the district had been emptied of its civilian population before the SDF entered, and IS had dug defensive trenches and tunnels in the area in a bid to hold off attacking forces.

Fighting was also continuing on the western outskirts of the city, the monitor said, adding that US special forces were actively participating in battles on several fronts.

Around 500 US military personnel, not all of them special forces, are believed to be participating in the battle for Raqa.

On Wednesday, an AFP correspondent saw coalition armoured vehicles parked among olive trees in the desert east of Raqa, covered with camouflage fabric.

SDF male and female commanders, dressed in fatigues and sporting colourful printed scarves wrapped around their heads, pored over maps on tablet devices to pinpoint targets.

Captured by the jihadists in 2014, Raqa has become synonymous with IS atrocities including beheadings and public displays of bodies, and also emerged as a hub for planning attacks abroad.

An estimated 300,000 civilians were believed to have been living under IS rule in Raqa, including 80,000 displaced from other parts of Syria.

But thousands have fled in recent months, and the UN humanitarian office estimates about 160,000 people remain in the city.

An activist from the anti-IS Raqa is Being Slaughtered Silently group told AFP on Wednesday that conditions in the city were deteriorating, describing continuous bombardment and water and electricity outages.

Civilians who have escaped have described harrowing journeys and being targeted by IS fighters as they fled.

The International Rescue Committee said earlier this week it was "deeply concerned for the safety of civilians in Raqa," noting a drop in the numbers fleeing the city in the past week.

That decrease could indicate IS intends to use remaining civilians "as human shields," the aid group said.

Civilians trapped in Raqa also face the risk of heavy coalition air strikes.

The number of reported civilian casualties in the coalition's strikes has swelled as the SDF offensive has intensified.

Along with Mosul in Iraq, Raqa was one of the twin pivots of the self-styled Islamic "caliphate" that IS declared nearly three years ago.

Iraqi forces backed by the coalition are battling IS in Mosul too. The jihadists there are now confined to a few neighbourhoods around the Old City.

The Syrian military has so far been absent from the battle for Raqa, though state news agency SANA said Thursday that the air force had targeted IS positions in the west of Raqa province.