French President Francois Hollande says he is heading to Ukraine with German Chancellor Angela Merkel to present a new peace initiative.
He said they would 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.
He said the new peace proposal was based on the “territorial integrity” of Ukraine and could be “acceptable to all” – but warned that diplomacy “cannot go on indefinitely”.
A spokesman for the Kremlin confirmed that President Putin would meet the two leaders on Friday to discuss “the fastest possible end to the civil war in south-eastern Ukraine”.
Ukraine and the West have accused Russia of arming rebels in eastern Ukraine and sending regular troops across the border – something Russia denies.
Separately, Mr Kerry is meeting President Petro Poroshenko and Prime Minister Arseniy Yatsenyuk in Kiev to discuss how the US can provide the country with assistance.
US President Barack Obama is said to be considering sending “defensive” weapons to Ukraine. The US is currently only providing “non-lethal” assistance.
Shortly after news emerged that the European leaders were to visit Ukraine, Russian Foreign Ministry spokesman Aleksander Lukashevich said any decision by the US to supply weapons to Ukraine would “inflict colossal damage to Russian-American relations”.