Beirut (AFP) - Kurdish and Arab fighters aided by US soldiers battled the Islamic State group north of its Syrian stronghold of Raqa as Iraqi forces edged towards the jihadist-held city of Fallujah.

The twin offensives are two of the most significant ground assaults against the extremists since they declared a self-styled "caliphate" straddling Iraq and Syria in 2014.

The assaults came as Syria's UN envoy said trapped civilians risk starvation unless Damascus and rebel groups allow greater access to humanitarian aid convoys.

The UN Security Council is to discuss Friday the humanitarian situation in Syria and the possibility of parachuting aid to besieged cities.

Near the front line north of Raqa city, an AFP photographer saw US soldiers on Wednesday assisting a Kurdish-Arab alliance known as the Syrian Democratic Forces.

The SDF is working its way through villages and farmland south of the town of Ain Issa, less than 60 kilometres (40 miles) from Raqa city.

It said its fighters had "advanced seven kilometres from Ain Issa and liberated five villages and four fields."

"We liberated the villages of Fatisah, Namroudiya, and Wastah as well as several fields. The coming battle will hold a lot of big surprises," SDF field commander Baraa al-Ghanem told AFP.

The Syrian Observatory for Human Rights said SDF fighters Thursday were shelling IS positions near Ain Issa as the US-led coalition carried out nearly non-stop air raids.

The Britain-based monitor updated its toll for the five-year war to more than 280,000 dead.

- Tunnel, car bombs -

A fragile truce agreed between the US and Russia in February had curtailed Syria's bloodshed despite consistent allegations of violations, but the international fight against IS was excluded from the ceasefire deal.

The first of 250 members of the US special operations forces were due to arrive this week in northeast Syria to support the campaign, joining dozens of advisers already on the ground.

An SDF field commander told AFP that US ground forces were "taking part" in operations north of Raqa.

"There are US forces using (anti-tank) TOW missiles to fire on the explosives-rigged cars that (IS) is using to attack the SDF," said Hawkar Kobane.

Asked about the men seen on the front line, US defence officials did not dispute that they were American special operations forces.

Observatory head Rami Abdel Rahman said IS is "concentrating 2,000 fighters along the front lines north of Raqa" to repel the SDF offensive.

"IS has prepared for this fight in recent months by digging tunnels and lining them with explosives, as well as preparing car bombs and hiding in buildings among civilians," Abdel Rahman added.

The SDF has insisted its current campaign is only for the rural area north of Raqa city -- but IS's de facto Syrian capital is expected to be the end goal.

"The ultimate purpose is Raqa city. It may not be short-term or mid-term, but besieging the city and blocking IS movements is also very important," said Washington-based analyst Mutlu Civiroglu.

The battle for Raqa province was announced on Tuesday with much fanfare, just one day after a major offensive began across the border for IS-held Fallujah.

Iraqi troops backed by pro-government militias have been advancing towards the city from surrounding areas.

As they close in, the UN's humanitarian coordinator in Iraq said she was receiving "distressing reports" of trapped civilians.

The UN said only 800 people of an estimated 50,000 had been able to flee Fallujah since May 22, "mostly from outlying areas".

The UN's Syria envoy Staffan de Mistura said "plenty of civilians" would starve across Syria unless aid deliveries are sped up.

Diplomats in New York said the envoy told the Security Council on Thursday that he has no plans to convene a new round of talks in the next two or three weeks.

More progress was needed to strengthen the ceasefire and deliver humanitarian aid before talks can resume, he said.

- 'Enormous' challenges -

The United States and its allies have set their sights on Raqa, Fallujah, and eventually Mosul in their bid to defeat IS.

But experts said ground efforts are likely to be drawn out and complicated.

"The challenges involved in weakening and dislodging the Islamic State from long-held fortified positions are enormous," wrote the New York-based Soufan Group.

To definitively defeat IS, Iraqi and Syrian fighters would have to address local concerns, sectarian politics, and ethnic divisions.

The Soufan Group said recapturing Fallujah "poses the biggest military challenge Iraqi forces have faced in the two years" since IS seized Mosul, their main Iraqi city.

In Syria, it wrote, IS fighters' "determination" to defend Raqa will make the fight to retake it "one of the fiercest yet".