Ukraine on Thursday brushed off the impeachment of Donald Trump an a US "internal issue", despite the process being triggered by a telephone conversation between the American leader and his Ukrainian counterpart.

Trump was impeached on Wednesday by the House of Representatives for abuse of power over the July conversation with President Volodymyr Zelensky.

Monday's summit aiming to end the conflict in eastern Ukraine will see Russia's Vladimir Putin face off with Ukraine's Volodymyr Zelensky for the first time.

Fourteen people remain missing from a fire that claimed a student's life in Ukraine's southern port city of Odessa, President Volodymyr Zelensky said Thursday.

Russia said Monday it had handed over three navy ships it seized a year ago from Ukraine, in the latest move to ease tensions between the two countries ahead of a crucial summit.

Ukrainian President Volodymyr Zelensky denied Thursday that Donald Trump tried to blackmail him by withholding military aid to Kiev in a phone call between the two leaders. 

All 24 Ukrainians captured by Russia last year are expected to be part of an imminent prisoner swap between Kiev and Moscow, the head of their defence team told AFP on Saturday.

Ukrainian President Volodymyr Zelensky on Wednesday said he had asked his Russian counterpart Vladimir Putin to put pressure on separatist fighters in the east, after four Ukrainian soldiers were killed in a grenade attack.

Two Ukrainian soldiers and one civilian were killed in incidents involving Kremlin-backed separatists, Kiev said Saturday, a day before the country votes in a parliamentary election.

Three months after winning a landslide victory in Ukraine's presidential election, former comedian Volodymyr Zelensky is hoping to consolidate his success in parliamentary polls on Sunday. 

An army helicopter has crashed in western Ukraine killing four servicemen, the military said on Thursday.