David Cameron has urged European Union leaders to stand firm on the need to maintain sanctions against Russia as signs of division emerged following the Ukraine peace deal.
The Prime Minister said it was "actions on the ground rather than just words on a piece of paper" that mattered, with the UK supporting a tough stance on the sanctions regime.
British officials said the apparent diplomatic breakthrough would be treated with caution until there was proof that Russian president Vladimir Putin had changed course.
Mr Cameron was in Brussels for a European Union summit, where the continent's leaders were given an update on the progress made by French president Francois Hollande and German chancellor Angela Merkel, who brokered the agreement struck at Minsk in Belarus which will see a ceasefire from Sunday.
The Prime Minister is understood to have been one of the leading opponents of a suggestion at the summit to weaken the sanctions regime against Russia by postponing the imposition of measures against 19 people due to take effect on Monday.
The Prime Minister had a face-to-face meeting with Ukrainian president Petro Poroshenko, who was invited to Brussels to address EU leaders on the situation.
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."
Mr 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, and British officials said they were concerned about the prospect of further bloodshed as both sides seek to gain ground before the ceasefire comes in to force on Sunday.
"An agreement has been signed, a ceasefire is coming in to play and one would hope that people could respect the agreement and the point at which it was signed," an official said.
" The context for Minsk is we have seen an upsurge in violence in recent weeks. Now we have a prospect of a ceasefire coming in on Sunday morning.
"But the rest for us is what happens on the ground. We are cautious, we need to learn the lessons of the last year."
The "devil will be in the detail here", the source added.
The source said it was "not a secret" that there had been resistance from some European countries to maintaining the economic penalties and " we, along with some others, had to work quite hard to make sure we took a decision now to renew the Crimean sanctions that were due to come up for rollover and to put some further names on the list".
At the summit Luxembourg raised the issue of sanctions against Russia in a sign of the divisions within Europe over how to respond to Mr Putin.
An EU diplomat said Mr Cameron was among those who resisted the effort to postpone the imposition of sanctions on 19 additional individuals, which are due to come in to effect on Monday, following the Minsk agreement.
The source said: "The ones that were agreed on Monday, that were supposed to come in to force next Monday, there was a bit of discussion whether you might see whether the ceasefire is holding and whether you need to do them ... the Luxembourgers were one (country) that kicked off some (talk of) 'should we start looking at sanctions'."
The Greeks also raised concerns about the wider approach to sanctions, the source indicated.
Mr Cameron resisted the proposal, while European Council president Donald Tusk was "firm" in his belief that the sanctions should stand, as was Polish prime minister Ewa Kopacz.
The diplomat said "we sort of saw this coming" and Ukrainian president Mr Poroshenko was "clear they should stay in place".
Mr Cameron used a first meeting with Greece's new prime minister Alexis Tsipras to stress the need for EU unity on the issue.
" The PM underlining the importance of maintaining EU unity and not rewarding Russian aggression by easing back on sanctions," a source said.
"Mr Tsipras shares the view that we need to look at what happens now post-Minsk. That is the test."
Mr Tusk said the EU would remain "cautious" about the prospects for peace in Ukraine "until words put down on paper translate into real deeds".
He said: " Europe will continue to remain united, both in times when there is need for a stronger reaction, and when there is a need for us to send bigger assistance to help rebuild Ukraine."
Shadow foreign secretary Douglas Alexander said: "President Putin has committed to cease support for armed groups in eastern Ukraine, and to facilitate the withdrawal of weapons and fighters from Ukrainian territory. His actions will be closely and carefully watched by the international community to ensure that he meets both the spirit and the letter of this deal."
As he left the summit Mr Cameron said: "What mattered today was that we sent a very clear message that unless Russian behaviour changed and Putin's behaviour changed then sanctions should stay in place. That's very much the message coming out of this Council meeting.
"What really matters now is that on the ground actual things happen rather than just words being said. That means heavy weapons need to be removed, it means a proper ceasefire has to be put in place, it means that people actually have to do the things they have signed up to do.
"If that happens it'll be progress but we should wait and watch to see whether that happens before we make a judgment."