KYIV, Ukraine, March 24, 2024 (BSS/AFP) - Ukraine's capital Kyiv and the western region of Lviv were under Russian air attack early Sunday, officials said, and Polish forces were also placed on heightened readiness.

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

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

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

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.

The RSZ also said "Polish and allied aircraft have been activated".

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.

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