Ukraine released a video of captured Russian soldiers on Tuesday, sharply escalating a dispute over Moscow's alleged backing for separatist rebels in the east of the former Soviet republic.

The footage was released only hours before the two countries' presidents were due to meet for the first time since June to discuss the conflict, which has killed more than 2,000 people and provoked Western sanctions against Russia.

In Moscow, a military source told Russian news agencies that a group of soldiers had surrendered to Ukrainian forces after crossing the border by accident.

Ukraine rejected that explanation. "This wasn't a mistake, but a special mission they were carrying out," military spokesman Andriy Lysenko said in a televised briefing.

He also said separatists were attacking the southeastern border town of Novoazovsk "at this very minute" and Ukrainian forces had destroyed 12 armoured infantry vehicles in the area.

Twelve Ukrainian service personnel had been killed in fighting in the past 24 hours, he said, including four border guards who died when Russian Mi-24 helicopters attacked a frontier post in Luhansk region on Monday.

Russia has always denied assertions by Ukraine, backed by the United States and the European Union, that it has been sending arms and troops across the border to support the pro-Moscow separatists.

The latest dispute cast a further pall over Tuesday's talks in the Belarussian capital Minsk, where presidents Vladimir Putin of Russia and Petro Poroshenko of Ukraine were meeting for the first time since a tense encounter in France on June 6.

"I hope the result of today's meeting will be the achievement of an agreement that will bring peace to Ukrainian soil," Poroshenko told reporters.

He held separate talks with EU foreign policy chief Catherine Ashton and Belarussian President Alexander Lukashenko, the summit host.

"CROSSED BY ACCIDENT"

Fighting in eastern Ukraine broke out in April, a month after Russia annexed Ukraine's Crimea peninsula in response to the toppling of a pro-Moscow president in Kiev.

Tuesday's video provided the strongest evidence yet to back up Kiev's claims of direct Russian military involvement, which Moscow has always disputed. It came a day after Ukraine's state security service said it had detained 10 Russian paratroopers who had crossed the border in a column of several dozen armed infantry vehicles.

In footage posted on the official Facebook page of the Ukrainian government's "anti-terrorist operation", the men were shown dressed in camouflage fatigues. One of them, who identified himself as Ivan Milchakov, listed his personal details, including the name of his paratroop regiment, which he said was based in the Russian town of Kostroma.

"I did not see where we crossed the border. They just told us we were going on a 70 km (45-mile) march over three days," he said. "Everything is different here, not like they show it on television. We've come as cannon fodder," he said in the video.

Another man in the footage, who gave his name as Sergeant Aleksei Generalov, said: "Stop sending in our boys. Why? This is not our war."

Social media accounts belonging to several of the men showed them in camouflage or paratroop uniform.

Russian news agencies quoted a defence ministry source as confirming that Russian servicemen had crossed into Ukraine but saying they did so inadvertently.

"The soldiers really did participate in a patrol of a section of the Russian-Ukrainian border, crossed it by accident on an unmarked section, and as far as we understand showed no resistance to the armed forces of Ukraine when they were detained," the source said.

REBELS ENCIRCLED

The Russian servicemen were detained, with their personal documents and weapons, near the small town of Amvrosiyivka in Donetsk region, the Ukrainian state security service said.

"Officially they are on military exercises in various corners of Russia. In reality they are involved in military aggression against Ukraine," Defence Minister Valeriy Heletey said in a Facebook post.

Since Putin and Poroshenko last met, Ukraine has turned the tide of the conflict and largely encircled pro-Russian rebels holding out in two eastern cities.

But the diplomatic crisis has only deepened, especially since the downing of a Malaysian airliner over rebel-held territory last month with the loss of 298 lives.

Stung by U.S. and EU sanctions against its finance, oil and defence sectors, Russia has hit back by banning most Western food imports, in a trade war that is hurting both the Russian and European economies. With tensions at their highest since the Cold War, Russian and NATO forces have both stepped up exercises in recent months.

Expectations for a breakthrough at Tuesday's talks were low. Reuters reporters in Donetsk, one of the two main rebel strongholds the Ukrainian military has been trying to retake, reported loud explosions and heavy machine-gun fire in the centre of the city overnight, but calm had returned by morning.

(This corrected version of the story fixes the first name of captured sergeant to Aleksei, 15th paragraph).

(Additional reporting by Richard Balmforth, Pavel Polityuk, Thomas Grove, Alexei Anishchuk and Anton Zverev; Writing by Mark Trevelyan; editing by David Stamp)