Fighting in eastern Ukraine left at least nine Ukrainian soldiers dead Friday, as government troops pressed their offensive against pro-Russian insurgents using heavy artillery and combat jets.

Ukrainian President Petro Poroshenko said Ukraine was ready to conduct another round of talks between representatives from Ukraine, Russia and the rebels on Saturday, but didn't name a venue. Two previous rounds of such negotiations held during a 10-day cease-fire have yielded no visible progress, but they brought the warring parties together for the first time.

Moscow strongly pushed for extending the truce and holding more talks in an apparent hope to negotiate a settlement that would allow it to secure a degree of influence over the neighbour. The continuing fighting is putting more pressure on Russian President Vladimir Putin, who has been facing increasingly angry nationalist demands to send troops to help the insurgency — a move that would inevitably trigger crippling Western economic sanctions.

Ukraine's National Security Council chief Andriy Parubiy said Friday that Russia was massing troops near the Ukrainian border and claimed that it let insurgents attack the Ukrainian border posts from its side. The statements could not be independently verified.

The two neighbours, who share a 2,000-kilometre border that is unmarked and unguarded outside of a few checkpoints, have been trading claims and counterclaims ever since Ukraine began fighting pro-Russia separatists in April.

The government said nine troops were killed and 13 others were wounded in Friday's fighting in the east, according to the Interfax news agency. It did not elaborate on where or how the deaths occurred.

Russians accused of arming rebels

Valentyn Nalyvaychenko, the head of Ukraine's SBU security service, told journalists that over the past four days 20 Russian tanks or armoured vehicles had illegally crossed the border to take part in the insurgency.

Ukraine says Russia is arming and supporting the separatists, a charge that Russia denies.

For its part, Moscow has repeatedly accused Ukrainian troops of firing shells that landed on the Russian side and on one occasion last month hit a Russian border post, wounding one customs officer. Russian border guards said 10 artillery shells flew from the Ukrainian side Friday, but no one was wounded.

It is nearly impossible to prove or dispel the claims from the opposing sides, since the fighting consists of intermittent clashes between small units with no fixed front line and it is deadly dangerous for journalists and other observers to travel around. Five journalists have been killed covering the conflict and several teams of observers from the Organization for Security and Co-operation in Europe have been kidnapped by rebels.

Russia has proposed hosting both OSCE observers and Ukrainian officers at its border checkpoints to prove it wasn't fuelling the mutiny.

Ukraine claims villages retaken

On Friday, Russia's Foreign Ministry accused Poroshenko's government of dragging its feet on holding another round of negotiations to resume a truce, and accused it of using cluster munitions against civilians.

Parubiy said government forces were attacking rebel positions in eastern Ukraine with artillery and planes, and that 17 villages had been recaptured since a unilateral cease-fire expired Monday. He said Ukrainian forces now controlled 23 of the 36 local regions within the Donetsk and Luhansk provinces that have declared independence.

Andrei Purgin of the separatist Donetsk People's Republic dismissed the government's report. "Maybe they occupied 17 villages of some sort, but there wasn't a single militia fighter there," he said.

Insurgents in Luhansk said Friday they have killed 125 Ukrainian troops, destroyed nine armoured vehicles and downed three combat aircraft since Wednesday.

Claims of enemy losses by both sides have proven exaggerated in the past.

Black Sea manoeuvres

The mutiny in eastern Ukraine, which erupted in mid-April after the ouster of the nation's pro-Moscow president and the subsequent Russia's annexation of Crimea, has killed more than 400 people, according to the United Nations.

The crisis in Ukraine has strained ties between Russia and the West to a degree unseen since the Cold War, prompting NATO to consider long-term responses to Moscow's apparent new willingness to flex its military might.

To that end, NATO and Russia both launched navy exercises Friday in the Black Sea.

NATO's drills in the western part of the Black Sea just a few hundred kilometres from Ukraine's coast involve ships from the U.S., Bulgaria, Greece, Romania and Turkey. They were to continue until July 13.

Russia's Black Sea Fleet fielded about 20 ships and more than 20 aircraft for its manoeuvres, which included missile launches at practice targets.