Ukrainian President Petro Poroshenko has urged President Vladimir Putin to strengthen Russian control over its borders to prevent militants and arms entering Ukraine after violence broke a truce there.
The cease-fire, declared by Poroshenko on June 20 to allow for peace talks with the pro-Russian rebels, is due to expire Monday, a deadline also set by EU leaders considering new sanctions against Russia.
The statement came Sunday after a four-way telephone conversation among the Ukrainian and Russian leaders, French President Francois Hollande and German Chancellor Angela Merkel, said a statement from Poroshenko's office.
"Ukraine called on the President of Russia to strengthen control over the Russian side of the state border in order to stop the penetration into Ukraine of militants and mercenaries and supplies of weapons and armored vehicles," it said.
The four leaders agreed to speak again Monday, the statement added.
The European Union has threatened more penalties on Moscow beyond existing asset freezes and visa bans unless pro-Russian rebels act to ease the crisis in eastern Ukraine by Monday.
Ukraine's National Guard said Sunday that rebels had used tanks and mortar shells to fire on a checkpoint near the separatist stronghold of Slovyansk, about 100 km (60 miles) from the border with Russia.
"There were no casualties among the military personnel there," its statement said. A spokesman for the operation told Channel 5 television that five soldiers had been killed in the past few days by rebel violence in violation of the truce.
Interfax news agency cited rebels as saying Ukrainian forces had shelled around Slovyansk, hitting a marketplace and an apartment building, causing injuries.