Russia accused the United States on Thursday of being behind what it says was a drone attack on the Kremlin intended to kill President Vladimir Putin.
Kremlin spokesman Dmitry Peskov made the allegation in a briefing to reporters, saying Washington should be aware that Russia knew it was selecting the targets and Ukraine was merely implementing US plans.
He did not provide any evidence to support the claim of US involvement.
Ukraine has denied involvement in the incident in the early hours of Wednesday, when video footage showed two flying objects approaching the Kremlin and one exploding with a bright flash.
The Kremlin has said it reserves the right to retaliate, but has not said what form this might take.
Peskov said Russia had several options and the response, when it came, would be carefully considered and balanced. He said an urgent investigation was under way, but could not say when the results would be known.