Russia demanded the release of two journalists on Saturday that were detained by Ukrainian security forces at a checkpoint in the restive east of the country.
"We demand that Kiev take immediate measures to free these Russian citizens without conditions," the Russian foreign ministry said in a statement.
The journalists -- sound operator Anton Malyshev and cameraman Andrei Sushenkov -- were working for Russia's Zvezda, or Star channel which is owned by the Russian defence ministry and airs patriotic programmes.
The reporters were handcuffed and questioned at a checkpoint outside the eastern Ukrainian city of Slavyansk on Friday before being taken away in a helicopter, Zvezda reported.
Ukraine's National Guard said Saturday it had detained the journalists "on suspicion of monitoring and collecting information" at the checkpoint.
The National Guard said in a statement on its website that the men were "handed over to the SBU (Ukraine's Security Service) to clarify the circumstances and purpose of their staying near the checkpoint."
The journalists were carrying press accreditation handed out by the self-proclaimed Donetsk People's Republic, the National Guard said.
The Russian foreign ministry called on Ukraine to "cease lawless acts towards teams of Russian and foreign journalists and ensure safe conditions for their professional activities."
The ministry said the journalists' detention on Friday "causes bafflement" since it came as "glimmers of hope are appearing for a de-escalation of the conflict in southeastern Ukraine."
Ukrainian troops in May detained two Russian journalists from a pro-Kremlin website and television channel Life News in the same region, later claiming they were transporting anti-aircraft missiles in their cars.
The Life News reporters were freed a week later following the intervention of Chechen leader Ramzan Kadyrov. The Kremlin awarded the journalists medals for valour.