Leaders of the G7 industrial nations have urged Russia to begin talks with the new leadership in Kiev to end the crisis in eastern Ukraine.
US President Barack Obama and UK PM David Cameron said Moscow must recognise Petro Poroshenko, who takes office as president on Saturday.
The G7 leaders meeting in Brussels said they were fully behind Mr Poroshenko.
Later in Paris Mr Cameron met Russian President Vladimir Putin, giving him a "very clear and firm set of messages".
The two leaders met in a customs area of the French capital's Charles de Gaulle airport.
This was Mr Putin's first face-to-face meeting with a Western leader since the Ukraine crisis began.
"The status quo, the situation today, is not acceptable and it needs to change," Mr Cameron said.
"We need the Russians to properly recognise and work with this new president. We need de-escalation, we need to stop arms and people crossing the border. We need action on these fronts."
Mr Putin later met French President Francois Hollande for dinner, but according to Russian media no announcements were made after the meeting. Mr Hollande was to see Mr Obama for a separate meal.
Hugh Schofield, BBC News, Paris
Desperate not to offend either of his mutually loathing invitees, the French president has taken the unusual step of agreeing to eat two meals in one evening.
First he dines on Thursday night at the Chiberta restaurant - just by the Arc de Triomphe - with President Obama.
Then - after presumably toying with his Michelin-starred plat and definitely not ordering the cheese - he returns to the Elysee for what they are calling "supper" with Vladimir Putin.
It is an unusual situation for a head of state - a reminder perhaps of the lengths Mr Hollande is prepared to go to make these two days a success.
When you are as unpopular at home as the president is, a big international shindig can come as a lifesaver. And shindigs rarely come as big as this one.
Mr Putin has not ruled out a meeting with Mr Poroshenko at a ceremony to mark the 70th anniversary of the D-Day landings in Normandy on Friday.
No talks are planned between Mr Putin and Mr Obama, who will also be at the ceremony.
Meanwhile, fighting continued in eastern Ukraine, with reports of an attack on a border post in Donetsk region.
A convoy of vehicles carrying pro-Russian separatists was involved in the attack near the Marynivka checkpoint which has now been repelled by government forces, the Ukrainian border guard service said.
The attack comes a day after rebels seized a border guard base in neighbouring Luhansk region after days of combat.
Speaking at a news conference in Brussels with Mr Cameron, Mr Obama said Russia should "seize the opportunity" provided by the change of leadership in Kiev, and be prepared to face sanctions if the situation continued to deteriorate.
"Russia needs to recognise that President-elect Poroshenko is the legitimately elected leader of Ukraine and engage the government in Kiev," he said.
"Given its influence over the militants in Ukraine, Russia continues to have a responsibility to convince them to end their violence, lay down their weapons and enter into a dialogue with the Ukrainian government.
"On the other hand, if Russia's provocations continue, it's clear from our discussion here that the G7 nations are ready to impose additional costs on Russia."
Earlier, German Chancellor Angela Merkel said that the G7 leaders had "exchanged expectations" about Ukraine and Russia.
"On substance, there is no difference whatsoever," she said. "There is great common ground."
European Commission President Jose Manuel Barroso said the group was united in sending a "resolute message" to Russia, that it should "recognise and fully engage with" the new Ukrainian authorities.
He added that Russia should "take concrete and credible measures to de-escalate the situation in the east of Ukraine".
The G7 summit is the first since Russia was expelled from the group following its annexation of Crimea in March.
On Thursday, leaders also discussed the global economic outlook, climate change and development issues.