Poroshenko's meeting will focus on political reforms demanded by Brussels in exchange for visa-free travel for Ukraine.
Ukrainian President Petro Poroshenko will press EU leaders on Thursday to hold firm against Russia amid fears that US President-elect Donald Trump may ditch the Western coalition against Moscow.
Poroshenko's summit with Donald Tusk and Jean-Claude Juncker, the European Union's top officials, will focus on political reforms demanded by Brussels in exchange for visa-free travel for Ukraine.
But he is also expected to push them to maintain sanctions against Russia over the annexation of Crimea and the conflict in eastern Ukraine at a time when Trump's election has thrown US support into doubt.
Trump's election victory has been met with trepidation in Kiev because of the billionaire TV star's praise of Russian President Vladimir Putin and the two men's vow to normalise ties after years of tension.
"I think the focus must be on the situation in Donbass (rebel-held eastern Ukraine), on Russian aggression, on the extension of sanctions," Ukrainian Foreign Minister Pavlo Klimkin told local television ahead of the summit.
"We have to force Russia to implement the Minsk agreement. For that we need our European friends and the representatives of the United States," he added.
The February 2015 Minsk accord was meant to bring an end to the conflict in eastern Ukraine between government forces and pro-Kremlin rebels but low-level fighting continues.
The West accuses Putin's Russia of continuing to support the rebels in a conflict which has claimed 10,000 lives in the past two-and-a-half years, a charge the Kremlin denies.
The EU is set at its next summit in December to discuss a six-month renewal of wide-ranging economic sanctions against Russia that were imposed after Malaysia Airlines flight MH17 was shot down over Ukraine in 2014.
But Ukraine now fears it could be left high and dry after Trump suggested earlier this year the US could accept Russia's annexation of Crimea if it led to improved relations between the two nations, which are bitterly at odds over Syria.
Outgoing US President Barack Obama, who discussed Ukraine with five EU leaders during a trip to Berlin last week, has said he hopes Trump will "stand up" to Russia when it breaches international norms, citing the conflicts in Syria and Ukraine.
The EU has however promised to stay loyal to Kiev, while Tusk emphasised to Trump in a phone call earlier this month the importance of "transatlantic cooperation" on Ukraine.
It was the growing closeness of former Soviet satellite Ukraine and the European Union, coupled with pro-EU protests that brought down former president Viktor Yanukovych, that angered Moscow in the first place.
In 2014 the EU and Ukraine finally signed a partnership agreement that Yanukovych had backed out of, which offers visa free travel to cement ties in exchange for Ukraine's pledges to act on corruption and to bring in political reforms.
A senior EU official said the summit would be an "opportunity to reaffirm the importance of our partnership on the basis of the values we share -- the rule of law, human rights, democracy."
The official added that Russia had a "qualified responsibility" for the situation in rebel eastern Ukraine.
"It is obvious that the separatists in Ukraine can continue their activities with a very strong support from outside of the country," the official said on condition of anonymity.
"That's why we believe Russia has a qualified responsibility for the security in the parts of the country that are not controlled by the government in Kiev."