Kyiv has struggled on the battlefield for months, weakened by desperately needed US military aid that is blocked in Congress and a shortage of men and ammunition.

Russia launched an aerial attack on five Ukrainian regions overnight and throughout Thursday morning, killing at least four people in the southern city of Mykolaiv, officials said.

Ukrainian President Volodymyr Zelensky urged the West not to "turn a blind eye" to Russia's aerial attacks and to provide more air defences as he visited Lithuania, one of Kyiv's staunchest allies against Russia.

He also signed a 10-year security cooperation agreement with Latvia, the latest in a number of similar agreements -- which are not mutual defence pacts -- with Western countries.

Back at home, the Ukrainian parliament, the Rada, adopted a mobilisation bill that was opposed for months by many in a country increasingly exhausted by war.

Facing pressure from army officials, lawmakers had a day earlier scrapped a clause that would have allowed soldiers fighting for more than 36 months to return home.

Soldiers at the front told AFP on Wednesday they were in "shock" about the demobilisation clause being ditched.

The bill, which needs to be signed into law by Zelensky, will strengthen punishments for draft dodgers and sets out new procedures for troop call-ups.

Russia's invasion of Ukraine has dragged on for more than two years, with no end in sight to fighting despite much of the front being virtually frozen.

- 'Completely destroyed' -

Zelensky said Moscow had fired more than 40 missiles and 40 drones at sites across Ukraine overnight.

He said Moscow was again targeting "critical infrastructure". Various regions reported power stations and gas distributors being hit.

Russia's defence ministry said it had launched what it called "retaliatory strikes" on Ukraine's energy facilities after a spate of attacks by Kyiv's forces on its own oil refineries.

Ukraine's southern command said at least four people were killed in an attack on Mykolaiv.

"The enemy continues its pinpoint ballistic strikes in the south of Ukraine.

"They insidiously hit Mykolaiv in the middle of the day," it said.

"According to preliminary information, four civilians were killed, five were wounded."

The Ukrainian city of Kharkiv -- which is being pounded on an almost daily basis at present -- was also attacked again, a day after a strike killed three people there.

Ukrainian Interior Minister Igor Klymenko described a "massive" attack that lasted "for several hours".

Ukraine's energy ministry said a strike had "completely destroyed" the Trypilska power station in the Kyiv region.

"All workers who were on shift during the shelling are alive," it said in a post on Telegram.

The mayor of Ukrainka, a town south of the capital, advised locals to shut their windows while firefighters sought to extinguish the blaze.

"I ask everyone to close the windows in their homes tightly so as not to breathe in harmful combustion products," Oleksandr Turenko said.

"Charge all devices. Make maximum water reserves."

- 'No sense' -

In Ukraine's westernmost region of Lviv, which borders the European Union, authorities said Russian troops attacked a gas distribution facility and an electricity substation.

Lviv region head Maksym Kozytsky said Moscow attacked the facilities with "cruise missiles of various classes and drones".

Russia, meanwhile, said it had destroyed 12 Ukrainian drones overnight, including three as far east as its Mordovia republic, more than 500 kilometres (300 miles) from the border.

Others were destroyed over the Kursk, Tambov, Belgorod, Bryansk and Lipetsk regions, it said.

Both countries have been firing dozens of drones at each other in waves of overnight attacks throughout the year, now in its third year.

In Moscow, the Kremlin criticised plans to hold a Ukrainian peace conference in Switzerland in June.

"We said many times that the process of (peace) talks without Russia makes no sense," Kremlin spokesman Dmitry Peskov told reporters.