Ukraine's Poroshenko Urges Putin to Tighten Borders after Violence

The ceasefire, declared by Poroshenko on June 20 to allow for peace talks with the pro-Russian rebels, is due to expire on Monday, a deadline also set by EU leaders considering new sanctions against Russia.

The statement came 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 armoured vehicles," it said.

The four leaders agreed to speak again on 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, Reuters reported.

Ukraine's National Guard said on Sunday rebels had used tanks and mortar shells to fire on a checkpoint near the separatist stronghold of Slaviansk, 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 Slaviansk, hitting a marketplace and an apartment building, causing injuries.

Poroshenko, under pressure from the West to keep up the ceasefire during talks with the rebels, is facing rising anger over the truce, which some Ukrainians say is only giving the rebels time to regroup and rearm.