Boris Johnson has urged action against the Syrian regime after a watchdog ruled an attack that killed more than 90 people used sarin nerve gas.

The foreign secretary said he had "no doubt" President's Bashar al-Assad's government was behind April's atrocity.

Russia blocked a UK-backed move at the United Nations to condemn the attack.

Mr Johnson is now appealing to world leaders to "unite behind the need to hold those responsible to account" by imposing further sanctions.

He was speaking after The Organisation for the Prohibition of Chemical Weapons (OPCW) found the deadly attack on Khan Sheikhoun, a town in Northern Syria held by rebel forces fighting the regime, on 4 April, could only be "determined as the use of sarin, as a chemical weapon".

The watchdog's report, which drew on samples from victims and survivors, did not reach any conclusion on who was responsible for the atrocity, which will be a matter for the UN to assess.

But Mr Johnson said it was the UK's belief that the Assad regime "almost certainly" carried out the attack.

He added: "I have absolutely no doubt that the finger of blame points at the Assad regime. Anyone who drops chemical weapons on innocent people should be held to account."

Mr Johnson failed to get his G7 counterparts to agree to targeted sanctions at a meeting in the aftermath of the attack - although the US has since imposed its own sanctions on individuals in the regime.

Mr Assad has dismissed the attack as a "fabrication" to justify US missile strikes while Russian president Vladimir Putin, a key ally of Assad, claimed it was "a provocation" staged "by people who wanted to blame" the Syrian leader.