Soldiers from NATO countries, under the guise of mercenaries, operate Kiev's air defense systems and rocket systems in Ukraine, said Sergei Rudskoy, first deputy chief of the General Staff of the Russian Armed Forces, Russia's TASS news agency reported on Monday.

"NATO military personnel, under the guise of mercenaries, participate in hostilities. They control air defense systems, tactical missiles and multiple launch rocket systems, and are part of assault detachments," the military official was quoted as saying.

There was no immediate comment from NATO countries about the claims.

Russian Foreign Minister Sergei Lavrov accused the United States and NATO of becoming directly involved in the conflict in Ukraine as they not only supply weapons but also train military personnel for Kiev in Britain, Germany, Italy and other countries.

The West has deployed hundreds of instructors to teach Ukrainians to use the supplied weapons, sent a huge number of mercenaries to Ukraine and provided intelligence, Lavrov said.