Theresa May and Donald Trump have agreed a “window of opportunity now exists” to persuade Russia that its links with Bashar Assad’s regime are no longer in its strategic interest as they discussed the situation in Syria.
The two leaders discussed the crisis by telephone as the US president thanked the PM for her backing of his military strikes against Syrian government forces last week.
The barrage of Tomahawk missiles damaged or destroyed 20 per cent of Syria’s operational aircraft, according to the US.
On Monday Mr Trump also spoke to German leader Angela Merkel, who voiced support for the intervention.
A Downing Street spokesman said: “The president thanked the Prime Minister for her support in the wake of last week’s US military action against the Assad regime.
“The Prime Minister and the president agreed that a window of opportunity now exists in which to persuade Russia that its alliance with Assad is no longer in its strategic interest.
“They agreed that US Secretary of State (Rex) Tillerson’s visit to Moscow this week provides an opportunity to make progress towards a solution which will deliver a lasting political settlement.”
The leaders also discussed wider issues affecting the Middle East, including “the threat posed by Iran throughout the region”, and the role of the international community and China in putting pressure on North Korea.
The White House said Mr Trump had called Mrs Merkel to discuss Syria and the regime’s use of “weapons of mass destruction against civilian men, women, and children.”
A spokesman said: “Prime Minister May and Chancellor Merkel expressed support for the action of the United States and agreed with President Trump on the importance of holding Syrian President Bashar al-Assad accountable.
“President Trump and his counterparts agreed to stay in touch regarding Syria and other international issues of mutual concern.”
The Prime Minister, who is on a short walking holiday in Wales, is being kept up to date on events in Syria.
Mr Trump ordered a series of missile strikes last week in response to the deaths of more than 80 people, including children, during a chemical attack in the rebel-held town of Khan Sheikhoun.
US Defence Secretary Jim Mattis said on Monday that the attack was a “measured response” to the use of chemical weapons.
“The Defence Department’s assessment is that the strike resulted in the damage or destruction of fuel and ammunition sites, air defence capabilities and 20% of Syria’s operational aircraft,” he said.
As a result the regime lost the ability to refuel or rearm aircraft at the airfield.
“The Syrian government would be ill-advised ever again to use chemical weapons,” Mr Mattis said.
The US president’s spokesman Sean Spicer warned on Monday that further action would be considered in certain circumstances.
“When you watch babies and children being gassed, and suffer under barrel bombs, you are instantaneously moved to action. I think this president has made it very clear that if those actions were to continue, further action will definitely be considered by the United States.”
The move came as Foreign Secretary Boris Johnson warned that senior Russian military officers involved in co-ordinating President Assad’s campaign of repression against his own people could face international sanctions.
At a meeting of G7 foreign ministers in Lucca, Italy, Mr Johnson issued a fresh appeal to Russian President Vladimir Putin to abandon his backing for his Syrian ally following last week’s chemical weapons attack.
He said if the Russians continued to support the regime in Damascus, they would be “contaminated” by its actions and could find themselves the target of new international sanctions.
“We will be discussing the possibility of further sanctions certainly on some of the Syrian military figures and indeed on some of the Russian military figures who have been involved in co-ordinating the Syrian military efforts,” Mr Johnson told reporters.
The Foreign Secretary said Mr Trump’s decision to launch cruise missiles against a Syrian air base in response to the regime’s use of sarin nerve agent had “changed the game” and the Russians had to decide which side they were on.
Mr Johnson defended his decision to pull out of talks with his Russian counterpart Sergey Lavrov, saying it was important that Mr Tillerson had the “clearest possible mandate” when he went to Moscow to deliver the response of the G7.
Russia and Iran - Assad’s two principal international backers - warned on Sunday that they would respond “with force” to any fresh attack on their ally.
Mr Trump’s son Eric said the US would not be “pushed around” by Mr Putin and the president is not intimidated by Moscow’s talk of war, the Daily Telegraph reported.
Mr Trump was influenced in his decision to strike Syrian government targets by the reaction to the chemical weapons attack from his daughter Ivanka, who was “heartbroken, and outraged”, his son said.
He said his father’s action on Syria showed he was not tied to Russia, as some critics claim.
Writing in the Daily Telegraph, Tory former foreign secretary Lord Hague, said the “sad truth” has dawned on President Trump that “Russia under Putin is not a reliable partner”.