David Cameron is to join fellow European leaders Francois Hollande and Angela Merkel in a conference call with Russian president Vladimir Putin in a bid to shore up the fragile Syrian ceasefire.
Downing Street said Friday's call will offer an opportunity for the Prime Minister, the French president and the German chancellor to press home to Mr Putin the importance of maintaining the truce, so that peace talks can go ahead in Geneva next week.
The start of the talks has already been pushed back by the United Nations from March 7 to March 9 to allow the ceasefire "to better settle down", amid reports of violations on all sides.
Moscow has been carrying out air strikes against what it terms "terrorists" in Syria since September, but Western nations have accused Mr Putin of also targeting more moderate groups opposing the regime of his protege Bashar Assad.
The call is the first time Mr Cameron will have spoken to Mr Putin since a UK inquiry found that the Russian president probably ordered the murder of former agent Alexander Litvinenko in London, but the PM's official spokeswoman said the bulk of the discussion will focus on Syria.
"It's an opportunity for the leaders of the UK, France and Germany to come together - in the way that they have before, when using the EU to put sanctions on Russia for its actions in eastern Ukraine - and make very clear to president Putin that we need this ceasefire to hold, to be a lasting one and to open the way for a real political transition in Syria," said the PM's spokeswoman.
"(It's about) wanting to make sure that the Russians fully support what they signed up to in the UN Security Council resolution at the end of last year to work towards peace in Syria."
The spokeswoman said the ceasefire in the five-year-old civil war, which came into force on Saturday, was "fragile".
"It is, as it were, a cessation of hostilities rather than a full ceasefire," she said. "That's why it's important that the international community keeps the pressure up on the regime and on the regime's backers, to fully respect it so that we can have genuine talks about peace, while also doing the work, that we are to engage with the moderate opposition in the role they need to play."
Mr Cameron told the House of Commons on Wednesday that the cessation of hostilities was "an important step forward, imperfect though it is, and it does enable the possibility of political negotiations starting next week".
He added: "Even though the ceasefire is imperfect, the fact that we have it is progress. Not every group is included in the ceasefire, but basically we are not seeing the attacks that were taking place on the moderate opposition, which is welcome.
"It has also enabled us, with others, to get aid to communities that desperately need it, including through air drops and convoys. I would not put too much optimism into the mix right now, but this is progress and we should work on it."