The deal aimed at calming tensions in Ukraine must be "rapidly implemented", David Cameron has said.
The European Union should continue preparations for potential extra sanctions against Russia although it will also do all it can to help bring in yesterday's deal, the Prime Minister and Herman Van Rompuy, president of the European Council, agreed in a phone conversation.
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."
Tensions remain high in eastern Ukraine after at least three died in a day of shooting during what was supposed to be an Easter truce.
In an exclusive interview with NBC News, the Ukrainian Prime Minister warns of the dangers of unchecked Russian expansion and aggression.
Russia, the US and the EU say that all sides have agreed to steps to "de-escalate" the crisis in Ukraine at talks in Geneva.