David Cameron has told Russian President Vladimir Putin the current situation in Ukraine is "not acceptable"
The prime minister said he gave the Russian president a "very clear and firm set of messages" during face-to-face talks in Paris about the crisis.
The hastily arranged meeting happened after the leaders of the G7 warned that Russia could face damaging economic sanction unless it changed course.
The Prime Minister said: "This was a meeting with a very clear and firm set of messages, which is that there is an opportunity for a successful, peaceful and stable Ukraine especially now there's been a presidential election.
"But the status quo, the situation today, is not acceptable and it needs to change."
Earlier at a joint news conference with US president Barack Obama following the G7 meeting in Brussels, Mr Cameron said Moscow needed to recognise the election of new Ukrainian president Petro Poroshenko, stop the flow of arms across the border and cease support for pro-Russian separatist groups.
Russia, which had been due to host a G8 summit in Sochi, was effectively expelled from the group it has been part of for almost two decades following its annexation of Crimea.
Instead the G7 held a summit in the Belgian capital.
Mr Cameron then flew from Brussels to Paris for his meeting with Mr Putin, which aides said was a chance to "look him in the eye" and stress the need for de-escalation.
After the hour-long discussion the Prime Minister said: "Russia needs to properly recognise and work with this new president, we need de-escalation, we need to stop arms crossing the border, we need action on these fronts but if that happens there is a diplomatic path that is open to have proper relations between Ukraine and Russia and a successful future for the people of Ukraine, which is what they deserve."
In Brussels Mr Cameron indicated that Mr Putin had one month to end Russia's destabilisation of the Ukraine or face crippling economic sanctions.
"If these things don't happen, then sectoral sanctions will follow," he said.
"The next month will be vital in judging if President Putin has taken these steps."
Mr Cameron met the Russian president at a secure area of Charles de Gaulle airport on the outskirts of Paris ahead of the gathering of world leaders for D-Day commemoration events in Normandy.
Mr Obama, who will also be in France for the 70th anniversary events, urged Mr Putin to "seize the opportunity" opened up by Mr Poroshenko's election in last month's presidential poll to negotiate a diplomatic resolution the crisis.
"We are at a point where Mr Putin has a chance to get back into a lane of international law. He has a president in Poroshenko who he can negotiate directly with," he said.
"It's clear that he recognises that Ukraine needs to have a good relationship with Russia, but also rightly affirms the right of Ukraine to engage with the rest of the world."
Mr Obama also echoed Mr Cameron in warning that the western powers could not afford to allow the situation to "drift" and Mr Putin needed to move quickly to show he was serious about finding a peaceful solution.
"The mere fact that some of the Russian soldiers have moved back off the border and that Russia is now destabilising Ukraine through surrogates rather than overtly and explicitly does not mean that we can afford three months or four months or six months of continued violence and conflict in eastern Ukraine," he said.
"We will have a chance to see what Mr Putin does over the next two, three, four weeks, and if he remains on the current course, then we've already indicated the kinds of actions that we're prepared to take."
While Mr Obama emphasised the importance of the G7 allies marching in "lockstep", he expressed his concern over French president Francois Hollande's decision to go ahead with a 1.2 billion euro (£1 billion) deal to supply two Mistral class helicopter assault ships to Russia.
"I have expressed some concerns - and I don't think I'm alone in this - about continuing significant defence deals with Russia at a time when they have violated basic international law and the territorial integrity and sovereignty of their neighbours," he said.
"President Hollande understands my position. I recognise that this is a big deal. I recognise that the jobs in France are important. I think it would have been preferable to press the pause button. President Hollande so far has made a different decision."