"I called him urgently. I told him that this brings us no closer to peace," Zelensky said during a briefing, adding that he had urged Putin to ask the Moscow-backed separatists to "stop killing our people".

"These aren't toys," Zelensky said.

The Ukrainian leader said he had a "long conversation" with Putin on Wednesday morning, only their second publicly announced phone call since former comedian Zelensky was elected in April.

Kiev reported Tuesday that four soldiers died in a rocket-propelled grenade attack, the highest daily toll since a truce went into force on July 21.

Zelensky at the time called the incident an attempt to "undermine" Ukraine's efforts toward peace and called on France, Germany and Russia to "meet as quickly as possible to resume negotiations".

Ukrainian forces and pro-Russian separatists, who carved out two "people's republics" in eastern Donetsk and Lugansk regions, have fought since April 2014, after the annexation of Crimea.

Some 13,000 people have been killed in the violence that Kiev blames on Putin.

Moscow has denied claims it has funnelled troops and arms across the border.

A series of truce agreements, brokered by France and Germany, have helped dampen hostilities but sporadic and sometimes serious clashes continue.