David Cameron has issued a warning to Russia's Vladimir Putin that EU sanctions will not be lifted unless the new ceasefire agreement in Ukraine is followed by "action on the ground".
The Prime Minister was speaking as he arrived in Brussels for an EU summit at which he will discuss the deal - struck in the early hours after all-night talks in Minsk - with Ukrainian president Petro Poroshenko, as well as German chancellor Angela Merkel and French president Francois Hollande, who brokered the agreement.
Mr Cameron said he welcomed the efforts made to bring an end to the conflict in eastern Ukraine, but insisted that the ceasefire must be "genuine".
"I welcome and thank Francois Hollande and Angela Merkel for the hard work that they have put in," said the Prime Minister. "If this is a genuine ceasefire, then of course that would be welcome.
"But what matters most of all is actually actions on the ground rather than just words on a piece of paper.
"I think we should be very clear that Vladimir Putin needs to know that unless his behaviour changes, the sanctions we have in place won't be altered."
Russian president Vladimir Putin announced the agreement following marathon talks in the capital of Belarus at the end of a week of diplomacy which has seen the French and German leaders engage in talks in Kiev and Moscow and Mrs Merkel fly to Washington to secure the support of US president Barack Obama.
It remains unclear whether separatist militias in the east of the country - who Mr Putin has always insisted are not under Russian control - will observe the conditions of the deal.
More than 5,000 people have died since April in fighting between Ukrainian forces and Moscow-backed rebels in two eastern provinces, and the bloodshed has risen sharply in recent weeks.
Mr Hollande said he and Mrs Merkel are committed to helping verify the ceasefire process with the Russian and Ukrainian leaders.
Speaking after the talks, the French president said the announcement of a ceasefire deal has come as a "relief to Europe".
Deputy Prime Minister Nick Clegg welcomed the ceasefire deal but warned that the international community must not "lower our guard".
"Any agreement, any statement from those leaders that they are going to enter in to a ceasefire is, of course, welcome," the Liberal Democrat leader said on his regular LBC radio phone-in.
"All I would say is that we have been here before. It is incredibly important that we keep everyone's feet to the fire, we don't lower our guard."
Referring to Mrs Merkel, he said: "I'm sure she would be the first to make sure that we are very vigilant to ensure that words are put in to action."