Russia's UN ambassador Vitaly Churkin said the latest attack put 'a very big question mark' over the truce's future.

The Syrian ceasefire was in danger of collapse last night after the US admitted 'unintentionally' killing about 80 government troops in air strikes.

The truce – in place since last Monday between the Russian-backed Syrian government and US-backed rebel groups – was designed to make both sides focus on fighting Islamic State.

It was hoped that would then lead to democratic elections and an end to the civil war.

But Russia's UN ambassador Vitaly Churkin said the latest attack put 'a very big question mark' over the truce's future.

The US expressed 'regret' for the 'unintentional loss of life', which it said came as it was attacking IS in the eastern province of Deir al-Zour on Saturday.

The US is not supposed to be assisting Syrian dictator Bashar al-Assad's forces against IS so it is unclear why it was launching air strikes in the area.

A Russian foreign ministry spokesman said the attack showed 'that the White House is defending Islamic State'. Syria's foreign ministry said the strike was 'planned and deliberate' and suggested 'pre-co-ordination between IS and the US forces'.

The air strikes were called off after the Russians notified the Americans they were hitting the Syrian military.

The ceasefire deal, brokered by Russia and the US, is meant to allow 'unimpeded and sustained humanitarian access' to besieged areas. They had agreed that, if the truce held for a week, Russia and the US would bomb militant Islamist groups together.