Jihadist fighters made a desperate last stand in eastern Syria on Wednesday, while their wives and children fled the final, blood-soaked implosion of their "caliphate".

The Syrian Democratic Forces on Saturday launched a final push to expel IS fighters from the sole remaining morsel of the proto-state they declared in 2014 across Syria and Iraq.

The United States will withdraw its troops from Syria, a US official told AFP on Wednesday, after President Donald Trump said America has "defeated ISIS" in the war-ravaged country.

The stunning move will have extraordinary geopolitical ramifications and throws into question the fate of US-backed Kurdish fighters who have been tackling Islamic State jihadists.

Sudan's president on Sunday became the first Arab League leader to visit Syria since civil war erupted there nearly eight years ago.

Omar al-Bashir was greeted at the Damascus airport by Syrian President Bashar Assad before they both headed to the presidential palace, where they held talks on bilateral relations and the latest developments in Syria and the region, according to the state-run news agency.

Syrian state media said Sunday that air defences had opened fire near Damascus airport, before withdrawing the report after what appeared to be a false alarm.

Iran's Revolutionary Guards said Monday they launched a missile strike against a "terrorist" headquarters in Syria in retaliation for an attack that killed 24 people in the Iranian city of Ahvaz.

Disagreement between Turkey and Russia over how to tackle the Syrian rebel stronghold of Idlib seems to have deferred a looming regime offensive on the province, analysts say.

Russia and Turkey are on opposite sides of the conflict, but key global allies.

Russia on Saturday said Syrian rebels are preparing a chemical attack in Idlib province which will be blamed on Damascus and used as a pretext for Western powers to hit government targets in the war-torn country. 

Moscow's accusation comes after US President Donald Trump's national security adviser John Bolton this week said Washington will respond "very strongly" if Syrian President Bashar al-Assad uses chemical weapons in an offensive to retake Idlib, one of the last rebel held provinces in the country. 

The rebels in southern Syria targeted by a major government offensive were long supported by Washington and seen as symbolizing relatively moderate opposition to Damascus.

The U.N. struggled on Friday to get a new round of Syrian peace talks off the ground, but with few signs of progress as dozens more civilian deaths underlined the scale of the challenge.

A dozen Syrian rebel factions have suspended talks on new peace negotiations, accusing President Bashar Assad's regime of violating a four-day-old ceasefire with attacks near Damascus that continued Tuesday.

The decision threatens the process sponsored by regime ally Russia and rebel backer Turkey, which began with a truce and is meant to lead to negotiations in the Kazakh capital Astana this month.