Russia accused Ukraine on Wednesday of attacking the Kremlin with drones overnight in a failed bid to kill President Vladimir Putin, Reuters reports.
The Kremlin said two drones had been used in the alleged attack on Putin's residence in the Kremlin citadel, but had been disabled by electronic defences.
It said Russia reserved the right to retaliate - a comment that suggested that Moscow might use the alleged incident to justify a further escalation in the 14-month-old war with Ukraine.
There was no immediate comment from Ukrainian authorities.
"Two unmanned aerial vehicles were aimed at the Kremlin. As a result of timely actions taken by the military and special services with the use of radar warfare systems, the devices were put out of action," the Kremlin said in a statement.
"We regard these actions as a planned terrorist act and an attempt on the president's life, carried out on the eve of Victory Day, the May 9 Parade, at which the presence of foreign guests is also planned.
"The Russian side reserves the right to take retaliatory measures where and when it sees fit."
It said fragments of the drones had been scattered on the territory of the Kremlin but there were no casualties or material damage.
RIA news agency said Putin had not been in the Kremlin at the time, and was working on Wednesday at his Novo Ogaryovo residence outside Moscow.
An unverified video circulating on Russian social media including the channel of the military news outlet Zvezda showed pale smoke rising behind the main Kremlin Palace in the walled complex after the purported incident.
