Washington DC - The US said Tuesday it was not involved in the killing of a senior Russian army officer in an operation claimed by Ukraine, but denounced his "atrocities."

"I can tell you that the United States was not aware of it in advance and was not involved," State Department spokesman Matthew Miller said of the killing of Igor Kirillov, who was the head of the Russian army's chemical weapons division.

But Miller pointed to earlier US assessments that Kirillov – the senior-most Russian military figure assassinated since Moscow invaded Ukraine – had ordered the use of riot control agents on the battlefield in violation of the Chemical Weapons Convention.

"He was a general who was involved in a number of atrocities. He was involved in the use of chemical weapons against [the] Ukrainian military," Miller said.

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Russian foreign ministry spokeswoman Maria Zakharova has accused Ukraine's Western allies of being "accomplices" to the brazen assassination in Moscow.

A US official, speaking earlier on condition of anonymity, said that the US "was not aware of the operation in advance, and we do not support or enable these kinds of activities."