Boris Johnson has offered Russia a chance to end the Syria crisis, saying “they still have time to be on the right side of the argument” as he warned that the United States may strike again.

The Foreign Secretary also branded Syrian president Bashar al-Assad an “arch-terrorist” and pledged that the UK and allied forces would gather evidence for “war crimes prosecutions for those responsible”.

Mr Johnson went on the front foot after he was criticised for failing publicly to persuade G7 countries to agree sanctions against Russia for its support of the Assad regime.

Relations with Moscow were strained when President Donald Trump ordered a missile strike against the Syrian airfield from which a chemical weapons attack was launched on civilians.

The gas attack had killed 70 people, many of them children who were filmed dead or dying, frothing at the mouth.

Mr Johnson then cancelled a visit to Moscow and threatening sanctions against Russia and Syria, a promise which backfired when the Germans and Italians refused to agree until an investigation has been carried out into who was to blame for the nerve gas attack in Idlib province.