French President Francois Hollande says he is heading to Ukraine with German Chancellor Angela Merkel to present a new peace initiative.
He said they will put forward the new plan before meeting Russian President Vladimir Putin in Moscow on Friday.
US Secretary of State John Kerry also landed in Kiev for talks on Thursday.
The diplomatic push to end the conflict, which has killed more than 5,000 people since last April, comes amid intensified fighting.
“Ukraine is at war. Heavy weapons are being used and civilians are being killed daily,” Mr Hollande said at a news conference on Thursday.
He said the new peace proposal was based on the “territorial integrity” of Ukraine, which could be “acceptable to all”.
A spokesman for the Kremlin confirmed that President Putin would meet with the two European leaders on Friday to discuss “the fastest possible end to the civil war in southeastern Ukraine”.
Ukraine and the West have accused Russia of arming rebels in eastern Ukraine and sending regular troops across the border but Russia denies this.
Mr Kerry is in Kiev to hold talks with President Petro Poroshenko and Prime Minister Arseniy Yatsenyuk as the US considers whether to send weapons to Ukraine.
The US has so far only provided “non-lethal” assistance to Ukraine.
But rather than arming Ukraine, Mr Hollande said there was another option: “That is the option of diplomacy – but that cannot go on indefinitely.”
The talks in Kiev come as Nato unveils details of a plan to bolster its military presence in Eastern Europe in response to the Ukraine crisis.
Nato chief Jens Stoltenberg says it will be the biggest reinforcement of its collective defence since the end of the Cold War.
A new rapid reaction “spearhead” force of up to 5,000 is expected to be announced, with its lead units able to deploy at two days’ notice.
Nato is also establishing a network of small command centres in Estonia, Lithuania, Latvia, Poland, Romania, and Bulgaria.
Mr Stoltenberg said the bloc was responding to “the aggressive actions we have seen from Russia, violating international law and annexing Crimea.”
Meanwhile, officials said on Thursday that the European Union is adding 19 people, including five Russians, to its sanctions list over the Ukraine crisis.
Nine “entities” will also be targeted by the sanctions, which were reportedly agreed at an emergency meeting of EU foreign ministers last week.
Fighting has intensified in eastern Ukraine in recent weeks amid a rebel offensive.
On Thursday, both government and rebel officials said several civilians had been killed in clashes over the past 24 hours. Ukraine’s military said five of its soldiers had died in the same period.
The fiercest fighting has been near the town of Debaltseve, where rebels are trying to surround Ukrainian troops. The town is a crucial rail hub linking the rebel-held cities of Donetsk and Luhansk.