Kyiv lodged an official note of protest with Moscow Tuesday over two Russian servicemen it says its forces captured in eastern Ukraine.
Ukrainian Foreign Ministry official Dmytro Kuleba, told reporters that the presence of Russian soldiers inside Ukraine was "a violation of all possible norms of international law" and raised doubts about Russia's commitment to the cease-fire agreement reached between Ukraine's government and Russia-backed separatists in February.
The two men were captured on Saturday and are recovering from wounds in a Kyiv hospital.
Russia's Defense Ministry says the two men had left active military service before they were captured inside Ukraine.
But Ukraine's military has released videos in which one of the two says they were part of a force of more than 200 men contracted by the GRU, the main intelligence directorate of the Russian armed forces' General Staff, that has been in Ukraine since March fighting alongside the rebels.
Ukraine says it plans to put the two men on trial on charges of terrorism.
President Vladimir Putin and other officials have consistently denied that Russian soldiers are in eastern Ukraine, insisting that any Russians fighting there are "volunteers."
Ukraine's military reported Tuesday that four more of its servicemen were killed in fighting in the eastern part of the country.