German Chancellor Angela Merkel and French President Francois Hollande will join Ukraine's new leader on a Sunday call to Russia's Vladimir Putin before Kiev's shaky truce with pro-Kremlin separatists expires.

The second such teleconference in four days was arranged in Brussels on Friday when Ukrainian President Petro Poroshenko put his name to a historic trade deal with Europe that breaks Kiev's bonds with Moscow.

The Organization for Security and Cooperation in Europe (OSCE) said on Saturday that pro-Russian rebels in eastern Ukraine have released four of the group's monitors they had detained at the end of May.

"Our Lugansk-based team of 4 monitors have been released after 1 month in captivity. Details to follow," the OSCE Special Monitoring Mission to Ukraine said on its Facebook page.

As his now-former comrades hugged their goodbyes and wished each other a safe journey home, Junior Sergeant Pavel Stupka explained why he refused to renounce his oath of allegiance to Ukraine.

"It would have been a betrayal. I took an oath to the Ukrainian people," Stupka said, pushing his beret back on his head.

Russia said Saturday that three shells fired by Ukrainian troops have hit a customs post and residential areas inside its territory, causing damage but no injuries.

"In the course of clashes on Ukrainian territory, the Ukrainian military fired shells which ended up on Russian territory," a spokesman for the border service in the Rostov region, Vasily Malayev, told Russian agencies.

Ukraine on Saturday sought concrete steps from Russia to back up a tenuous truce it extended with pro-Moscow rebels in the hope of calming a deadly insurgency sparked by its new westward course.

President Petro Poroshenko returned triumphant from Brussels on Friday having opened the way to Ukraine's eventual membership in the European Union by signing the final chapters of a landmark free trade and political association accord.

A new round of Western sanctions on Russia over the crisis in Ukraine could seriously impact its already stalled economy, Economy Minister Alexei Ulyukayev said Saturday.

The minister said Russia has prepared for three possible scenarios in the event of tougher economic sanctions.

EU leaders on Friday gave Russia three days to change policy on Ukraine or face the prospect of tougher sanctions as Kiev said it would consider a ceasefire extension with pro-Moscow rebels.

In a statement Friday, the 28 European Union leaders said Moscow had to take four steps, among them clear backing for the Ukraine government's peace plan, by Monday June 30.

Some 110,000 people have fled to Russia from Ukraine while more than 54,000 have been displaced inside the conflict-torn country, the U.N. said Friday.

"Since the start of 2014, 110,000 Ukrainians have arrived in Russia," Melissa Fleming, spokeswoman for the U.N.'s refugee agency, told reporters.

Swiss President and OSCE chief Didier Burkhalter on Friday hailed the release of four of the organization's observers abducted in east Ukraine a month ago, but demanded the freeing of others still being held.

Burkhalter expressed "his gratitude to all who helped in setting four of the eight monitors of the OSCE Special Monitoring Mission free," his office said in a statement.

Pro-Russian rebels launched an attack on Thursday on a Ukrainian irregular troops unit in the heart of the eastern industrial city of Donetsk, an Agence France Presse reporter at the scene said.

At least 200 gunmen surrounded a compound housing a unit of Ukraine's National Guard and opened fire, despite a temporary truce that had been accepted by both sides.