A SERIOUS split has opened up between Washington and London over Syria after Theresa May refused to back any new air strikes on tyrant Bashar Assad.

The US government has threatened to launch a fresh round of missile strikes on the regime if it uses nerve gas again.

4 Theresa May refused to back US threat of further airstrikes Credit: Reuters

Donald Trump’s National Security Adviser HR McMaster and his ambassador to the UN Nikki Haley have both warned the US is “prepared to do more” to enforce an international red line on chemical weapons.

But Downing Street refused to back President Trump’s tough stand when repeatedly pressed today.

The Prime Minister’s official spokesman would only say the question was “hypothetical”, adding: “Our focus is on building international support for a political solution to end the conflict and bring lasting peace and stability to Syria”.

4 It is a serious split between Washington and London over the issue Credit: PA

No10 also said Theresa May has no plans to go back to the House of Commons to ask for approval for the UK to stand alongside the US and join future strikes on Assad.

A vote by MPs in December 2015 only authorises the RAF to join attacks on ISIS in Syria.

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Questions are also being asked by MPs why Theresa May has not spoken to Donald Trump about the security crisis yet.

Almost a week has elapsed since it erupted when 100 civilians were killed when the town of Khan Shaykhun was targeted with Sarin gas last Tuesday.

4 The PM has not spoken to Trump - but has called Canadian counterpart Justin Trudeau Credit: AP

Mrs May did ring Canada’s Justin Trudeau on Sunday night to work with him to “build support for a political solution”.

Mr Trudeau, who also runs a powerful G7 country, is seen as a foreign policy dove and could be a key ally in trying to restrain Donald Trump.

Downing Street also said the two leaders “agreed on their support” for the initial US cruise missile strikes on Friday, dubbing them “an appropriate response”.

US Secretary of State Rex Tillerson reinforced America’s new intention to act against war crimes yesterday.

4 US Secretary of State Rex Tillerson reinforced America’s new intention to act against war crimes Credit: Reuters

While commemorating a 1944 Nazi massacre in Italy ahead of a summit of G7 foreign ministers, Mr Tillerson said: “We rededicate ourselves to holding to account any and all who commit crimes against the innocents anywhere in the world”.

Former Tory PM David Cameron failed to persuade the Commons to allow cruise missile strikes on Assad to punish an earlier chemical weapons attack that killed 1,000 civilians in 2013.