Ukraine has said that its forces have captured 10 Russian soldiers in the east of the country but Moscow has claimed the soldiers crossed the border accidentally.
The development comes as the president of Russia, Vladimir Putin, and his Ukrainian counterpart Petro Poroshenko were getting ready to meet on Tuesday in Belarus in the presence of EU foreign affairs chief.
The Facebook page for the Ukraine's anti-rebel operation, which includes the military, the national guard and interior ministry forces, said on Tuesday that the soldiers were from a Russian paratrooper division and were captured on Monday in the area of Amvrosiivka, near the Russian border in the Donetsk region.
The posting did not give details of how the capture took place. It posted videos of five men it said were among the captives, one of whom said the soldiers had been told they were being mobilised to take part in military exercises.
Russian news agencies quoted an unnamed source in the Russian defence ministry as saying that the soldiers were patrolling the border area and probably crossed the border 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," a source from Russia's defence ministry was quoted by Russian news agency RIA Novosti as saying on Tuesday.
Ukraine said on Tuesday that Russian helicopters had attacked a border post in Luhansk region the previous day, killing four border guards.
The Ukrainian troops have been fighting pro-Russia rebels in the east of the country for around four months.
Kiev and the US have repeatedly accused Moscow of stoking the rebellion, but this is the first time Ukrainian authorities claimed to have taken captive soldiers from Russia's regular army.
On Monday, Ukraine said 10 tanks and two armored vehicles belonging to the Russian military crossed into southeastern Ukraine bearing the flags of the self-proclaimed Donetsk People's Republic.
On the same day, the Ukrainian president dissolved parliament and called for early elections on October 26.
"Many deputies who are in the Rada [parliament] are direct sponsors or accomplices, that is to say allies of the militant separatists," Poroshenko said in a statement posted on his website.
Tuesday's talks, expected to begin after 1100 GMT in the Belarussian capital of Minsk, will include European Union foreign policy chief Catherine Ashton and the leaders of Belarus and Kazakhstan, partners in a Russian-led customs union.
Since Putin and Poroshenko last met on June 6 in France, Ukraine has turned the tide of the conflict and largely encircled pro-Russian rebels holding out in two cities in the east of the former Soviet republic.