Ukraine's security service has arrested 10 Russian citizens with backgrounds in intelligence, according to the Ukrainian State Security Service (SBU).
"We have about 10 Russians, with Russian passports, who have been detained," said an SBU spokeswoman.
"They have all had experience of intelligence work," she said.
The arrests come as talks between Russia, Ukraine, the United States and the European Union began in Geneva.
The location and reasons for their arrest remain unclear.
Russian president Vladimir Putin earlier claimed that allegations of Russian military intervention in eastern Ukraine were "total nonsense", in a state-television interview.
"They are locals who are involved there. Those from Kiev do not talk to the people in the east of Ukraine. A solution can be found only through a proper dialogue," he said.
"People in the east of Ukraine started arming themselves. Instead of finding a diplomatic solution, Kiev decided to use force," he continued.
In other developments, three pro-Russian militants were killed in clashes with Ukrainian security forces at a National Guard base in southeastern Ukraine.
Ukraine's interior ministry said a crowd of 300 men attacked a base in the Black Sea port of Mariupol, leaving servicemen inside the base with no option but to fire upon the attackers.