German Chancellor Angela Merkel and French President Francois Hollande have arrived in Ukraine’s capital Kiev to present a new peace initiative.
Secretary of State John Kerry, who is also in Kiev, said the US wanted a diplomatic solution, but would not close its eyes to Russian aggression.
Fighting between Ukrainian forces and pro-Russian rebels has killed more than 5,000 people since last April.
Ukraine and the West have accused Russia of arming rebels in eastern Ukraine and sending regular troops across the border.
Russia denies direct involvement but says some Russian volunteers are fighting alongside the rebels.
Speaking at a joint news conference with John Kerry, Ukrainian PM Arseniy Yatsenyuk said: “We need to get peace. But we will never consider anything that undermines territorial integrity… of Ukraine.”
John Kerry accused Russia of violating Ukraine’s sovereignty, saying that Russia had been acting with “impunity”, crossing the Ukrainian border “at will with weapons [and] personnel”.
“We are choosing a peaceful solution through diplomacy – but you cannot have a one-sided peace,” he said.
John Kerry added that President Barack Obama was still “reviewing all options”, including the possibility of providing “defensive weapons” to Ukraine, due to the dangerous escalation in violence.
The US is currently only providing “non-lethal” assistance.
Russian Foreign Ministry spokesman Alexander Lukashevich said any decision by the US to supply weapons to Ukraine would “inflict colossal damage to Russian-American relations”.
Several senior Western officials have also expressed concern at the prospect of US arms being sent to Ukraine.
German Foreign Minister Frank-Walter Steinmeier likened the option to “throwing more weapons on the bonfire”, while NATO commander Philip Breedlove said governments must take into account that the move “could trigger a more strident reaction from Russia”.
Francois Hollande and Angela Merkel arrived in Kiev on February 5, in what appeared to be a speedily arranged visit.
They met Ukrainian President Petro Poroshenko, who thanked them for their visit at “a very urgent time”.
Francois Hollande had said that he and Angela Merkel would present a new peace proposal based on the “territorial integrity” of Ukraine, which could be “acceptable to all”.
However, he warned that diplomacy “cannot go on indefinitely”.
Angela Merkel and Francois Hollande will meet Russian President Vladimir Putin in Moscow on February 6.
A spokesman for the Kremlin said President Vladimir Putin would discuss “the fastest possible end to the civil war in south-eastern Ukraine”.
Correspondents say it is not clear how the latest attempt will differ from previous, aborted peace efforts – but there is speculation that Francois Hollande and Angela Merkel hope to discourage the US from supplying Ukraine with weapons.
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.
A new rapid reaction “spearhead” force of up to 5,000 troops 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 centers in Estonia, Lithuania, Latvia, Poland, Romania, and Bulgaria.
Meanwhile, officials said on February 5 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.
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.
Some 1.2 million Ukrainians have fled their homes since last April, when the rebels seized a big swathe of the Luhansk and Donetsk regions following Russia’s annexation of Crimea.
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