Russia accused Ukraine on Wednesday of a failed attempt to assassinate President Vladimir Putin in a drone attack on the Kremlin citadel in Moscow, and threatened to retaliate.

Kyiv denied any role in the reported incident, with a senior aide to President Volodymyr Zelenskiy calling it a sign the Kremlin was planning a major new attack on Ukraine.

Shortly after the Kremlin announcement, Ukraine reported alerts for air strikes over Kyiv and other cities.

"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."

It said fragments of drones were scattered in the Kremlin grounds but there were no injuries or damage. Putin himself was safe.

"The Russian side reserves the right to take retaliatory measures where and when it sees fit," the statement said.

Video posted by Baza, a Telegram channel with links to Russia's law enforcement agencies, showed a flying object approaching the dome of a Kremlin building overlooking Red Square, exploding in a burst of light just before reaching it.

Other video posted on a neighbourhood internet group showed a plume of smoke over the Kremlin's gold domes. Reuters could not independently verify the videos.

In comments sent to Reuters, Ukrainian presidential advisor Mykhailo Podolyak said: "Of course, Ukraine has nothing to do with drone attacks on the Kremlin. We do not attack the Kremlin because, first of all, it does not resolve any military tasks."

Podolyak said the accusation, along with a separate announcement that Russia had caught suspected saboteurs in Crimea, "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 Kyiv's promised spring counter-offensive.

Scores of firefighters battled a huge fire that Russian authorities blamed on a Ukrainian drone crashing into an oil terminal on Russia's side of its bridge to occupied 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 on fire. Ukraine said it had shot down 21 of 26 Iranian-made drones in an overnight volley, shielding targets in the capital Kyiv where air raid sirens blared for hours through the night.

Zelenskiy was greeted by cheering crowds in Finland's capital Helsinki, where he arrived on a rare trip abroad to meet Nordic leaders, some of Ukraine's strongest supporters.

Ukraine and Russia have both been launching long-range strikes since last week in apparent anticipation of Ukraine's upcoming counter-offensive, which Zelenskiy said would begin soon.

After a lull of nearly two months, Russia began a new wave of long range attacks with missiles last Friday, including one that killed 23 civilians while they slept in an apartment building in the city of Uman hundreds of miles from the front.

Since then, a suspected Ukrainian drone strike caused a fire at a Russian oil terminal in occupied Crimea, Russia hit dozens of homes and an industrial enterprise in Ukraine's Dnipropetrovsk region, and blasts derailed freight trains in Russia's Bryansk region adjacent to Ukraine two days in a row.

Moscow says its long-range attacks have struck military targets, though it has produced no evidence to support this. Kyiv, without confirming any role in incidents in Russia or occupied Crimea, says destroying infrastructure is part of preparation for its planned ground assault. (Reuters)