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.