Russian President Vladimir Putin stressed on Saturday "the unacceptable nature of the military-biological activities of the United States in Ukraine" in a phone conversation with Luxembourg's Prime Minister Xavier Bettel, the Kremlin said in a statement.

Such activities pose a huge danger to both Russia and the whole Europe, Putin said.

The two leaders discussed Russia's military operation in Ukraine, and Putin noted the missile attacks by the Ukrainian security forces on cities in Donbass, said the statement.

Bettel said that "we must put an end to" the conflict, and "no one stands to gain from these clashes; not Russia, not Europe and certainly not Ukraine," according to a statement on the official website of the Luxembourg government.

Bettel briefed Putin on contacts with leaders of Ukraine and other countries, and Putin outlined his assessments of the Russian-Ukrainian peace talks, the statement added.

The Russian Foreign Ministry on Thursday asked Washington to provide all information on the activities of U.S.-linked biological laboratories in Ukraine.

Last week, U.S. Undersecretary of State for Political Affairs Victoria Nuland testified before a Senate Foreign Relations Committee hearing on Ukraine, admitting "Ukraine has biological research facilities."