Kyiv, Ukraine - Russian missile and drone strikes battered Ukraine's power grid on Monday, killing at least four people and forcing authorities to introduce emergency blackouts.

Officials said 15 regions across the country were targeted in the aerial assault, which began during the night and was the biggest in weeks.

The attacks come as Ukraine presses a major cross-border offensive into Russia's Kursk region, where Kyiv has been battling for nearly three weeks and claimed on Sunday to be advancing.

"Russian terrorists have once again targeted energy infrastructure. Unfortunately, there is damage in a number of regions," Ukrainian Prime Minister Denys Shmygal said.

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State-owned electricity system operator Ukrenergo was forced to introduce emergency power cuts to stabilize the system following the barrage, while train schedules were disrupted.

Explosions from what appeared to be air defenses could be heard in the capital Kyiv early on Monday, while residents rushed to take shelter in subway stations, AFP journalists reported.

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

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

The Russian defense ministry said it had struck energy infrastructure used to support Ukraine's defense industry.

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

The attacks on Monday killed four people and wounded more than a dozen across the country, officials said.

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The governor of the central Dnipropetrovsk region, Sergiy Lysak, said Russian forces had attacked "en masse."

"There is one dead, a 69-year-old man," the governor wrote on social media.

In the southern Zaporizhzhia region, the attack killed one civilian, local governor Ivan Fedorov said.

In the western city of Lutsk, Russian bombardment damaged an apartment building and an infrastructure facility, killing one person and injuring five others, mayor Igor Polishchuk said.

And in the central region of Zhytomyr, one person was killed and several others were wounded, authorities said.

Russia also attacked railway infrastructure in the northern Sumy region, injuring a man and damaging buildings, national operator Ukrainian Railways said.

"Some railway stations, which were also cut off from power due to the outage in the city's networks, have been switched to backup generators," it said.

The attack targeted energy facilities across the country, including the southern Odesa region, the wider Kyiv region and the region of Lviv in the west of the country, authorities said.

"As a result, there are partial power outages in (the city of) Lviv and the region," governor Maksym Kozytskyi said on social media.

Four people were wounded in a missile attack on the southern Odesa region, including a 10-year-old boy, governor Oleg Kiper said.

In the neighboring southern region of Mykolaiv, "massive rocket fire" wounded three other people, the governor Vitaliy Kim said.

Earlier, an attack on an industrial facility in the eastern region of Poltava wounded five people, governor Filip Pronin said.

"The enemy is once again terrorizing the whole of Ukraine with missiles. The energy sector is in the crosshairs," energy minister German Galushchenko said.

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

Authorities in the eastern Kharkiv region meanwhile said one resident had been killed on Monday morning by Russian rocket fire but it was not immediately clear whether that incident was part of the missile and drone barrage.