David Cameron has insisted agreements aimed at calming tensions in Ukraine must be implemented rapidly, Downing Street said tonight.
The European Union should continue preparations for potential extra sanctions against Russia although it will also do all it can to help bring in today's deal, the Prime Minister and Herman Van Rompuy, president of the European Council, agreed in a phone conversation.
The pair spoke after the US, EU, Ukraine and Russia came to a tentative agreement in Geneva, which could put on hold economic sanctions the West had prepared to impose on Russia if the talks were fruitless.
Mr Cameron is due to speak to US president Barack Obama shortly, Number 10 added.
A Downing Street spokesman said of Mr Cameron's conversation with Mr Van Rompuy: "They agreed that the outcome of the Geneva meeting represented a positive step forward, but that it was essential that the agreements reached were rapidly implemented.
"They agreed that the European Union should do all it could to help implement these agreements, but in the meantime should continue preparatory work on potential additional sanctions, so that the EU was ready if the agreement was not implemented and the situation on the ground in Ukraine deteriorated."
The agreement, reached after seven hours of negotiation, requires all sides to refrain from violence, intimidation or provocative actions.
It calls for the disarming of all illegally armed groups and for control of buildings seized by pro-Russian separatists during the protests to be turned back over to authorities.
It also gives amnesty to protesters who comply with the demands, except those found guilty of committing capital crimes.
Monitors with the Organisation of Security and Co-operation in Europe (OSCE) will be tasked with helping Ukraine authorities and local communities comply with the requirements in the agreement.
Kiev's plans to reform its constitution and transfer more power from the central government to regional authorities must be inclusive, transparent and accountable - including through the creation of a broad national dialogue.
Foreign Secretary William Hague echoed Mr Cameron's view as he welcomed the commitments in the talks.
He said: "The steps contained in the joint agreement offer a route for de-escalation.
"But rapid implementation of the agreement is critical, particularly the commitments by Russia that all illegally armed groups must be disarmed, all illegally seized buildings must be returned to legitimate owners, and all illegally occupied streets, squares and other public places in Ukrainian cities and towns must be vacated.
"The OSCE monitoring mission, which is already deployed on the ground, will have an essential role in verifying the implementation of this agreement.
"I also welcome the commitment to a transparent, inclusive constitutional process and national dialogue."
He added it was for Ukrainians to decide how their country is governed, citing next month's elections as a "crucial first step".
Shadow foreign secretary Douglas Alexander MP said today's deal would be a "welcome step forward" if implemented.
He went on: "Both Russia and Ukraine have a responsibility to deliver on their commitments to help resolve the ongoing tensions and ensure that stability is restored.
"The days ahead will be a crucial test of Russia's stated resolve to help stabilise the situation, but this initial step is potentially a sign of progress."