Russia fired hundreds of drones and missiles at Ukraine on Monday, killing at least four people and battering the country's already weakened energy grid, officials said.

The Russian attack triggered widespread blackouts and came after Kyiv claimed new advances in its incursion in Russia's Kursk region.

Ukraine's air force confirmed early Tuesday the "takeoff of several Tu-95MS from the Engels airfield" in western Russia, prompting air raid alerts across the country.

Two more people were killed overnight in the central Ukrainian city of Kryvyi Rig, according to a local official, in strikes that also damaged homes and businesses.

On Monday, President Volodymyr Zelensky said Moscow launched at least 127 missiles and 109 drones in "one of the largest Russian attacks".

Of those, 102 missiles and 99 drones were shot down, according to Ukrainian Air Force Commander Mykola Oleshchuk, who described it as Russia's "most massive" attack.

The United States and Britain both condemned the assault, with US President Joe Biden calling it "outrageous" and British Foreign Secretary David Lammy branding it "cowardly".

Germany's foreign ministry said that "once again, Putin's Russia is saturating Ukraine's lifelines with missiles".

- 'Very scary' -

State-owned electricity supplier Ukrenergo announced emergency power cuts to stabilise its system following the barrage, while train schedules were disrupted.

Residents in the capital Kyiv rushed to take shelter in metro stations early Monday, as AFP reporters heard the booms of what appeared to be air defences.

"We are always worried. We have been under stress for almost three years now," said Yulia Voloshyna, a 34-year-old lawyer taking refuge in the Kyiv metro.

"It was very scary, to be honest. You don't know what to expect," she said.

Since invading in February 2022, Russia has launched repeated large-scale drone and missile attacks on Ukraine, including punishing strikes on energy facilities.

The Russian defence ministry confirmed it hit energy facilities in a statement, claiming that they were being used to aid Ukraine's "military-production complex".

The attacks early on Monday killed at least four people and wounded over 20 people across the country, officials said.

Two others were killed in later strikes during the day, according to authorities.

- Poland airspace 'violation' -

NATO member Poland said its airspace was violated during the barrage, probably by a drone.

"We are probably dealing with the entry of an object on Polish territory. The object was confirmed by at least three radiolocation stations," General Maciej Klisz, operational commander of the armed forces, told reporters.

Army command spokesman Jacek Goryszewski said it was "highly likely that it could have been a Shahed-type drone" of Iranian design, used by the Russian military.

"But this has to be verified," he told AFP, adding that it could not be ruled out that the drone had already left Polish territory.

Zelensky called for European air forces to help Kyiv down drones and missiles in the future.

"In our various regions of Ukraine, we could do much more to protect lives if the aviation of our European neighbours worked together with our F-16s and together with our air defence," Zelensky said in an address.

Andriy Yermak, Zelensky's chief of staff, said the attack showed that Kyiv needed permission to strike "deep into the territory of Russia with Western weapons".

Zelensky said Ukraine's surprise cross-border incursion into Russia's Kursk region launched on August 6 "is, among other things, a way to compensate for the lack of range".

On Sunday, he said that the surprise manoeuvre had yielded further advances, albeit small ones.

- 'Saddened' -

Monday's aerial barrage came after a safety advisor working for the Reuters news agency, Ryan Evans, was killed in a missile strike on a hotel in eastern Ukraine late Saturday.

Britain's Lammy said he was "deeply saddened to learn" of his death.

Six of the agency's crew covering the war were staying at the hotel in Kramatorsk, the last major city under Ukrainian control in the Donetsk region.

The Kremlin said there was "still no clarity" about the strike when asked about Zelensky's assertion that the attack was carried out "deliberately".

"I will say it again. The strikes are against military infrastructure targets or targets related to military infrastructure," Kremlin spokesman Dmitry Peskov said.

Zelensky said defending the logistics hub of Pokrovsk, also in the Donetsk region, was "most difficult" with Ukraine strengthening its positions there.

Over the border, one person died and six others were injured in a fire at an oil refinery in the Siberian city of Omsk on Monday, said regional governor Vitaly Khotsenko.

Authorities did not specify the source of the fire.

Russian media reported that loud explosions were heard near the refinery, operated by Russian oil giant Gazprom and about 2,300 kilometres (1,430 miles) from Ukraine.

Ukraine regularly carries out drone attacks on oil and gas infrastructure in Russia, sometimes far from its border.

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Gazprom