Ukraine's capital Kyiv and the western region of Lviv came under a "massive" Russian air attack early Sunday, officials said, and Polish forces were also placed on heightened readiness.

Russia and Ukraine have been engaged in a series of deadly aerial attacks, with Sunday's strikes also coming a day after the Russian military said it had seized the Ukrainian village of Ivanivske west of Bakhmut.

A militant attack on a Moscow concert hall on Friday that killed at least 133 people also became a new flashpoint between the two arch-rivals.

"Explosions in the capital. Air defence is working. Do not leave shelters," Kyiv mayor Vitali Klitschko posted on Telegram on Sunday.

Lviv regional governor Maksym Kozytskyi said Stryi district, south of the city of Lviv, near the Polish border, was also attacked.

Ukraine was earlier placed under a nationwide air alert that warned of cruise missiles being launched from Russian Tu-95MS strategic bombers. The alert was lifted about two hours later.

Sergiy Popko, head of the Kyiv city military administration, said the missiles were fired at the capital "in groups" in the third pre-dawn attack in four days.

Preliminary reports suggested there were no casualties or damage, he said, and the city's air defences had hit "about a dozen" missiles.

"The enemy continues massive missile terror against Ukraine," Popko said on Telegram. "It does not give up its goal of destroying Kyiv at any cost."

US Ambassador to Ukraine Bridget Brink also noted the increased frequency of recent attacks.

"Russia continues to indiscriminately launch drones and missiles with no regard for millions of civilians, violating international law," Brink wrote on social media platform X.

- Drones, missiles -

In Lviv, Mayor Andriy Sadovy said about 20 missiles and seven Iranian-made Shahed drones were fired at the region.

"They targeted critical infrastructure facilities," Sadovy said.

The Polish Armed Forces Operational Command (RSZ) said its forces were on a heightened state of readiness because of the "intensive long-range aviation activity of the Russian Federation tonight" and the missile attacks in Ukraine.

"All necessary procedures to ensure the security of Polish airspace have been activated and the RSZ is monitoring the situation continuously," it said.

Russia and Ukraine have increased their air attacks in recent weeks.

Kyiv, which has struggled to find weapons and soldiers after more than two years of war, has promised to retaliate by taking the fighting to Russian soil.

Multiple air attacks Saturday on the Russian border region of Belgorod adjoining Ukraine killed two people and injured at least seven, the regional governor said.

Further east, a drone attack on the Samara region caused a fire at a major oil refinery, the latest in a series of strikes against Russia's energy industry.

Belgorod governor Vyacheslav Gladkov wrote on Telegram that two districts in his region, as well as the regional capital, Belgorod, had been hit in drone and air attacks.

A man was killed when three balconies on an apartment building collapsed, Gladkov said.

Russia said later Saturday that it had repulsed a barrage of Ukrainian missiles fired at the city of Sevastopol in Crimea, which it annexed in 2014.

Sevastopol's governor Mikhail Razvozhayev said rocket fragments had killed a 65-year-old resident and four other people had been wounded.

"It was the biggest attack in recent times," he said.

- Territorial gains -

Moscow has escalated its own strikes, firing dozens of missiles on Friday and launching dozens of explosive drones to destroy Ukraine's energy infrastructure.

Russian forces have also taken control of a string of frontline settlements in recent weeks.

The capture last month of Adviivka, near the Russian-held stronghold of Donetsk, was the first major territorial gain made by Russia since the devastated city of Bakhmut was seized 10 months ago.

Russian President Vladimir Putin hailed that success as a sign that Russian forces are back on the offensive.

Putin has also sought to tie Kyiv to the Moscow concert hall attack, saying four "perpetrators" were detained while travelling towards Ukraine.

Kyiv has strongly denied any involvement, saying that Russia was looking for excuses to step up the war.

The United States has said it has seen no sign of Ukrainian involvement in the Crocus City Hall attack.