Russia has accused Ukraine of a failed attempt to assassinate President Vladimir Putin in a drone attack on the Kremlin citadel in Moscow.
Ukrainian President Volodymyr Zekenskiy said his country had nothing to do with the reported incident.
"We don't attack Putin, or Moscow, we fight on our territory," Zelenskiy told a press conference during a visit to Finland, alluding to Russian forces that swept into Ukraine last year.
A senior aide to Zelenskiy called the accusation a sign that the Kremlin was planning a major new attack on Ukraine.
Shortly after the Kremlin announcement, Ukraine reported alerts for air strikes over the capital Kyiv and other cities.
Russia said that 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," a Kremlin statement said.
"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."
Fragments of drones were scattered in the Kremlin grounds but there were no injuries or damage, it said.
Putin himself was safe.
The RIA news agency said he had not been in the Kremlin at the time, and was working on Wednesday at his Novo Ogaryovo residence outside Moscow.
"The Russian side reserves the right to take retaliatory measures where and when it sees fit," the Kremlin added.
Former Russian president Dmitry Medvedev, now deputy head of Russia's Security Council, said the incident "leaves us no option but to physically eliminate Zelenskiy and his clique".
Two of numerous videos published on Russian social media channels show two objects flying on the same trajectory toward one of the highest points in the Kremlin complex, the dome of the Senate.
The first seemed to be destroyed with little more than a puff of smoke, the second appeared to leave blazing wreckage on the dome.
Reuters could not independently verify the videos.
Ukrainian presidential adviser Mykhailo Podolyak said the drone accusation, along with an announcement that Russia had caught suspected saboteurs in Ukraine's Russian-occupied Crimea region, "clearly indicates the preparation of a large scale terrorist provocation by Russia in the coming days".
Elsewhere, oil depots were ablaze in both southern Russia and Ukraine, as both sides escalated a drone war ahead of Ukraine's promised spring counteroffensive against Russian forces.
Scores of firefighters battled a huge conflagration that Russian authorities also blamed on a Ukrainian drone crashing into an oil terminal on Russia's side of its bridge to Crimea.
A fuel depot in Ukraine was ablaze after a suspected Russian drone strike on the central city of Kropyvnytskyi.
An administrative building in Ukraine's southern Dnipropetrovsk region was also hit by a drone and set ablaze.
Ukraine said it had shot down 21 of 26 Iranian-made drones in an overnight volley, shielding targets in Kyiv where air raid sirens blared for hours through the night.
Sixteen people were killed by Russian shelling in Ukraine's southern Kherson region, 12 of them in Kherson city, on Wednesday, the Ukrainian prosecutor's office said.
Russian forces have regularly shelled the city from parts of the region occupied by Russia.
Ukraine and Russia have both been carrying out long-range strikes since last week in apparent anticipation of a Ukrainian counteroffensive, which Zelenskiy said would begin soon.
Russia says it has struck military targets although it has produced no evidence to support this.
Ukraine, without confirming any role in incidents in Russia or Crimea, says destroying infrastructure is preparation for its planned ground assault.
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