Diplomatic efforts are under way to end renewed fighting in eastern Ukraine.
French President Francois Hollande and German Chancellor Angela Merkel held urgent talks in the capital Kiev, presenting a new peace initiative.
US 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 nearly 5,400 people since April, the UN says.
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.
‘Acting with impunity’
President Poroshenko said in a statement the talks with Ms Merkel and Mr Hollande “give hopes… for a ceasefire”. He earlier also thanked the German and French leaders for their visit at “a very urgent time”.
The German and French leaders made no public comments after Thursday’s meeting, in what appeared to be a speedily arranged visit.
Ahead of the talks, Mr Hollande said he and Mrs 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”.
Mr Hollande and Mrs Merkel will now meet Russian President Vladimir Putin in Moscow on Friday.
Speaking earlier at a joint news conference with Mr Kerry, Ukrainian Prime Minister Arseniy Yatsenyuk said: “We need to get peace. But we will never consider anything that undermines territorial integrity… of Ukraine”.
Mr 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,” Mr Kerry said.
He added that Mr 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, like night-vision goggles and body armour.
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”.
Analysis: Jonathan Marcus, BBC diplomatic correspondent
Should the West arm Ukraine? Might such a move – by giving Ukraine more effective defences – reduce the risk of a major escalation?
Or might it simply add fuel to the fire, encouraging Moscow to mount a major offensive westwards to punch through to the territory it already holds in the Crimea?
Nato countries are increasingly divided on this crucial question.
The consensus view up to now has been not to supply weapons, but to give non-lethal support like radars, medical gear, body-armour and so on. Only Poland has explicitly stated that it might arm Ukraine if asked.
Now the balance of the debate is shifting in the US too, but still there is no clear will to provide weapons to the Kiev government. There is a growing sense of urgency and concern within Nato and if diplomacy fails, arming Kiev may become a serious option.
A spokesman for the Kremlin said Mr 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 Mr Hollande and Mrs 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 centres in Estonia, Lithuania, Latvia, Poland, Romania, and Bulgaria.
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.
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.