An air strike on a Syrian village held by the Islamic State group has killed at least 25 people, all of them civilians, according to activists.

The delivery of humanitarian aid to half a million displaced civilians in north eastern Syria is under threat from fighting between Iraqi forces and Kurdish fighters near the border, a medical charity has warned.

The bodies of at least 65 civilians have been found in a central Syrian town retaken by government forces from the Islamic State group, activists have reported.

US-backed Kurdish-led forces fighting Islamic State militants in Syria have said they are waging the "final" battle to uproot the extremists from Raqqa, once the terror group's de facto capital.

The Russian military has denied claims that it struck a US-backed force in eastern Syria, wounding six fighters.

Syrian rebels say they have killed 20 army soldiers in a tunnel blast in the battle for the north-eastern suburbs of Damascus.

At least eight people have been killed and 12 others injured in a series of car bomb explosions in the Syrian capital Damascus as officials claimed they had foiled a plot to target crowded areas.

An air strike targeting an Islamic State-run jail in eastern Syria has killed at least 42 prisoners, activists said.

A US-backed Syrian force has captured a neighbourhood of the Islamic State group's de-facto capital of Raqqa after two days of fierce fighting.

An attack by Islamic State on an area held by the Syrian government in southern Aleppo province has been repulsed by troops and allied fighters, the military reports.

The Britain-based Syrian Observatory for Human Rights reported that air strikes had seriously damaged the medical centre in Abdin village, in Idlib

The evacuation of more than 3,000 Syrians from four areas as part of a population transfer has been postponed a day after a deadly blast killed more than 120 people, many of them government supporters.

LATEST: At least 100 people have been killed in the blast near evacuation buses outside Aleppo city, the Syrian opposition rescue service says.

The Syrian government and the opposition have begun a co-ordinated population swap of tens of thousands of people from four besieged towns.

US Ambassador Nikki Haley warned on Wednesday that US President's Trump administration will take action against chemical attacks in Syria - if the UN Security Council fails to act.

A suspected government chemical attack in an opposition-held town in northern Syria has killed dozens of people.

US-backed forces in northern Syria have paused military operations near a dam held by the Islamic State group to allow engineers to fix any problems.

An airstrike on a mosque in a rebel-held area of northern Syria today killed at least 35 people, emergency officials and activists said.

Syrian government warplanes have pounded a rebel-held neighbourhood in the central city of Homs, killing at least three people and wounding dozens, opposition activists said.

Islamic State militants launched a new attack on the besieged city of Deir el-Zour today - in the most fierce fighting in a year, the Syrian government and opposition activists said.

The death toll in the car bomb attack on a busy market in a Syrian rebel-held town along the Turkish border today has risen to 43, activists and rescue workers said.

The UN Security Council unanimously adopted a resolution supporting efforts by Russia and Turkey to end the nearly six-year conflict in Syria and jump-start peace negotiations.

Russia, Iran and Turkey are ready to act as guarantors in a peace deal between the Syrian government and the opposition, Russian foreign minister Sergey Lavrov said.

Ten buses carrying civilians from two Shiite villages besieged by rebels in northern Syria are on their way to government-controlled areas, a Syrian activist group and a Lebanon-based TV station have said.

France has struck a compromise with Russia on a United Nations resolution that it said would prevent "mass atrocities" in besieged areas of Aleppo, Syria, where thousands of trapped civilians and rebel fighters await evacuation in freezing temperatures.

A deal to evacuate trapped civilians and fighters in war-ravaged east Aleppo and two Syrian villages has been thrown into doubt after assailants torched six buses assigned to the operation.

An agreement has been reached to allow "humanitarian cases" to leave two besieged government-held Shiite villages in north-western Syria, a step which would allow the evacuations of civilians and rebels from eastern Aleppo to resume, Hezbollah's media arm and a monitoring group have said.

Evacuations from eastern Aleppo remain suspended until rebels allow residents of two besieged Shiite villages to leave to government-controlled areas, according to Syrian state TV.

A ceasefire "surrender deal" allowing civilians and opposition fighters to be evacuated from eastern Aleppo has begun as Russia declared all military action had stopped and the Syrian government assumed control of the former rebel enclave.

Some 50,000 civilians have fled eastern Aleppo over the past two days in a "constant stream", Russia says, as Syrian government forces close in on the last pocket of opposition control.

Russian foreign minister Sergey Lavrov has warned Syrian rebels in Aleppo that they will be wiped out unless they agree to stop fighting and quit the besieged city.

Thousands of residents have been fleeing eastern Aleppo after simultaneous advances inside the divided city by Syrian government and Kurdish-led forces.

Nearly 10,000 civilians have fled eastern Aleppo, with Government forces determined to retake the entire city.

Syrian government forces have regained control of areas they lost over the past two weeks to a rebel offensive on the edge of the northern city of Aleppo.

Syrian government forces have launched a counter-offensive under the cover of air strikes in an attempt to regain control of areas they had lost to insurgents the day before in Aleppo, activists and state media said.

Turkish jets and artillery have struck US-backed Syrian Kurdish fighters in northern Syria, killing as many as 200 fighters, according to Turkey's state-run news agency.

America has dismissed an eight-hour pause in attacks by Russian and Syrian forces on rebel-held eastern districts of Aleppo as "too little, too late".

Shelling and more than a dozen airstrikes on rebel-held parts of Aleppo have reportedly killed at least 11 people in Syria.

A top commander in Syria's rebranded al Qaida affiliate who was close to terror network leader Ayman al-Zawahri has been killed in an air strike, the group said.

Russia has warned the US against carrying out any attacks on Syrian government forces, claiming there would be repercussions across the Middle East.

Russia and Iran will be guilty of war crimes if they do not put pressure on Syrian President Bashar Assad to stop escalating violence in the country, says France's foreign minister.

Four staff have been killed in an air strike on a medical facility in Syria, a relief group has said.

A ceasefire brokered by the United States and Russia is set to begin in Syria at sunset today.

At least 13 people have been killed in a string of bomb attacks inside government territory in Syria, according to state media.