The ceasefire in Syria will only succeed if the Syrian regime changes course and Russia ends attacks on civilians and the moderate opposition, Downing Street said, following talks between Western leaders including David Cameron and Barack Obama.
The Prime Minister, the US president, French president Francois Hollande and German chancellor Angela Merkel discussed the situation following the latest developments in the peace process.
The agreement brokered by Washington and Moscow will see a new ceasefire take effect on Saturday, although military efforts will continue against the Islamic State militants, Syria's al Qaida branch and any other militia designated as a terrorist organisation by the UN Security Council.
The Western leaders believe the ceasefire could be "an important step towards peace" but the only way to end the "suffering of the Syrian people" was for an end to Bashar Assad's rule.
The leaders also agreed on the importance of maintaining efforts to tackle IS, also known as Daesh.
A Downing Street spokeswoman said on Tuesday: "The Prime Minister held a video teleconference on Syria earlier today with president Obama, chancellor Merkel and president Hollande.
"They welcomed today's cessation of hostilities agreement, which could be an important step towards peace in Syria.
"They noted the agreement would only succeed if there was a change of behaviour by the Syrian regime and its backers, with Russia in particular ending its attacks on Syrian civilians and moderate opposition groups.
"They also agreed on the vital importance of the Syrian regime allowing humanitarian access.
"They agreed a political solution and transition away from Assad was the only way to end the suffering of the Syrian people, and that a credible political transition process therefore had to get under way as soon as possible.
"They also agreed on the importance of the global coalition fight against Daesh continuing at pace, to sustain momentum in recovering ground.
"On migration, they agreed they were firmly committed to the Nato migration mission, noting the importance of Greece and Turkey working together and with the international community to ensure the mission's success."