Ukraine launched its biggest drone attack on the Russian capital on Tuesday with at least 91 drones targeting Moscow, killing at least one person, sparking fires, closing airports and forcing dozens of flights to be diverted, Russian officials said.

A total of 337 Ukrainian drones were downed over Russia, including 91 over the Moscow region and 126 over the Kursk region, where Ukrainian forces have been pulling back, the defense ministry said.

The massive dawn drone attack unfurled just as a team of Ukrainian officials prepare to meet a U.S. team in Saudi Arabia to seek grounds for possible peace talks in the three-year-old war, and as Russian forces try to encircle thousands of Ukrainian soldiers in the western Russian region of Kursk.

As rush hour built, Moscow Mayor Sergei Sobyanin said air defenses were still repelling attacks on the city, which along with the surrounding region has a population of at least 21 million and is one of the biggest metropolitan areas in Europe.

"The most massive attack of enemy UAVs (unmanned aerial vehicles) on Moscow has been repelled," Sobyanin said in a post on Telegram.

Moscow Region Governor Andrei Vorobyov said at least one person was killed and three injured, and he posted a picture of a wrecked apartment with its windows blown out.

Vorobyov said that some residents were forced to evacuate a multi-storey building in the Ramenskoye district of the Moscow region, about 50 km southeast of the Kremlin.

There was no sign of panic in Moscow, commuters went to work as normal in central Moscow.

Russia's aviation watchdog said flights were suspended at all four of Moscow's airports to ensure air safety after the attacks. Two other airports, in the Yaroslavl and Nizhny Novgorod regions, both east of Moscow, were also closed.

Though U.S. President Donald Trump says he wants to deliver peace in Ukraine, the war is heating up on the battlefield with a major Russian spring offensive in Kursk and a series of Ukrainian drone attacks deep into Russia.

Russia has developed a myriad of electronic "umbrellas" over Moscow and over key installations, with additional advanced internal layers over strategic buildings, and a complex web of air defences to shoot down the drones before they reach the Kremlin in the heart of the capital.

Kyiv, itself the target of repeated mass drone strikes from Russian forces, has tried to strike back against its vastly larger eastern neighbor with repeated drone strikes against oil refineries, airfields and even Russian strategic early-warning radar stations.

The war, the biggest in Europe since World War II, has combined grinding World War One style attrition trench and artillery warfare with the major innovation of drones.

Moscow and Kyiv have both sought to buy and develop new drones, deploy them in innovative ways, and seek new ways to destroy them - from using farmers' shotguns to advanced electronic jamming systems.

Both sides have turned cheap commercial drones into deadly weapons while ramping up their own production.

Soldiers on both sides have reported a visceral fear of drones - and both sides have used macabre video footage of fatal drone strikes in their propaganda, with soldiers shown being blown apart in toilets or running from burning vehicles.

Russian President Vladimir Putin, who has sought to insulate Moscow from the rigours of the war, has called Ukrainian drone attacks that target civilian infrastructure such as nuclear power plants "terrorism" and has vowed a response.

Moscow, by far Russia's richest city, has boomed during the war, buoyed by the biggest defence spending splurge since the Cold War.