President Petro Poroshenko is expected to be joined by German Chancellor Angela Merkel and French President Francois Hollande on the scheduled phone call to the Russian leader on the eve of the lifting of the ceasefire.
Sunday's teleconference is primarily meant to check on any visible shift in Moscow before the European Union and Washington consider unleashing biting sanctions against Russia's financial and defence sectors the following day.
Both Ukraine and its Western allies have been seeking concrete steps from Russia to back up the ceasefire Kiev extended with the militias on Friday in the hope of calming a deadly insurgency sparked by the country's new westward course.
The call also comes after pro-Kremlin rebels in eastern Ukraine on Saturday released four monitors from the Organisation for Security and Cooperation in Europe (OSCE) -- one woman and three men -- after being pressed by Putin to meet the terms of the tenuous truce with Kiev.
The OSCE observers looked tired but relieved as they were handed over by heavily-armed rebels to one of the group's representative at a hotel in the eastern hub city of Donetsk.
"We are releasing the last four observers who were being held on the territory of the Lugansk People's Republic," the prime minister of the self-proclaimed Donetsk People's Republic told reporters.
"We consider that we have fulfilled all our obligations," Oleksandr Borodai said.
- Ceasefire 'only the start' -
The first group of observers detained on May 26 in the Donetsk province was handed over to the OSCE on Thursday.
The Vienna-based organisation said the second team includes nationals from Germany, the Netherlands, Spain and Russia.
"We welcome the return of the last four of the missing OSCE special monitoring teammates after a month away," deputy mission head in Ukraine, Mark Etherington, said in a statement.
"The detention of OSCE monitors has substantially constrained the operations of the mission in eastern Ukraine at a time when a flow of objective information has never been more important."
The OSCE -- a 57-nation body created in the 1970s to oversee European security during the Cold War -- has played a central role in trying to mediate an end to a 12-week insurgency convulsing the ex-Soviet state.
Germany also commented on the release of the OSCE team, expressing relief and stressing the need to maintain the ceasefire.
The extended ceasefire "is a positive sign but it is only the start of a process," said German Foreign Minister Frank-Walter Steinmeier in a statement.
"The weapons must be silenced in a lasting way to give negotiations a chance of succeeding," he said.
- Monday truce deadline -
After signing a landmark trade and political association accord with the European Union on Friday, Poroshenko decided to extend the fragile ceasefire until Monday evening.
He added the condition that Russia requires the insurgents to return border crossings to Ukrainian forces and set up a monitoring mechanism for a long-term truce to halt the rebel uprising that has already claimed 450 lives.
A spokesman for Ukraine's eastern campaign told AFP Saturday that the past day of fighting had seen three soldiers killed and six others wounded outside the rebel stronghold city of Slavyansk.
"Everyone knows that a bad peace is better than a good war," Defence Minister Mykhailo Koval told Ukraine's UNIAN news agency.
Putin has publicly backed the ceasefire's extensions and promoted direct talks between Poroshenko and top rebel commanders.
But the West wants the Kremlin chief to call on the fighters to lay down their weapons and relinquish control of state buildings they had seized across a dozen eastern cities and towns.
EU leaders agreed at their Brussels summit Friday "to reconvene at any time to adopt further significant restrictive measures if a detailed list of concrete steps are not taken by Russia and the separatists by Monday".
The United States stressed that it was also ready to act at any point.
But public statements in Russia have thus far suggested that it was busy preparing an economic counter-offensive against Ukraine that would put up prohibitive barriers to its trade.
Foreign Minister Sergei Lavrov said on Saturday that Moscow would treat Ukraine and the ex-Soviet states of Georgia of Moldova, which also signed EU trade deals on Friday, "based on one criterium -- how (the agreements) might hurt Russian trade".
Russian and EU ministers have tentatively agreed to meet on July 11 to discuss how Moscow's concerns might best be addressed.