Ukraine said Russia had destroyed homes in the southeast and knocked out power in many places with a new round of missile attacks as the West imposed a price cap on Russian seaborne oil to try to limit Moscow's ability to finance its invasion.

A new barrage had been expected for days and came on a day when emergency blackouts were due to end, with previous damage repaired. The strikes plunged parts of the country back into freezing darkness with temperatures across Ukraine now firmly below zero.

They followed reported explosions at two air bases inside Russia overnight, both hundreds of miles from Ukraine. One of them, the Engels base in Saratov region, houses bomber planes that are part of Russia's strategic nuclear forces.

Three people were killed when a fuel tanker exploded at the air base in Ryazan, 185 km (115 miles) southeast of Moscow, state news agency RIA said.

Saratov regional governor Roman Busargin reassured residents they were safe after what he called reports on social media of a "loud bang and a flash" at the Engels air base.

"Information about incidents at military facilities is being checked by law enforcement agencies," he said, without elaborating.

The Engels base, about 730 km (455 miles) south of Moscow, is one of two strategic bomber bases housing Russia's air-delivered nuclear capability, comprising 60-70 planes.

Previous mysterious blasts have damaged arms stores and fuel depots in Russian regions near Ukraine and knocked out at least seven warplanes in Crimea, annexed by Russia from Ukraine in 2014.

President Vladimir Putin drove a Mercedes across the bridge linking southern Russia to Crimea on Monday, less than two months since that too, was hit by an explosion.

Kyiv has not claimed responsibility for any of the blasts, saying only that such incidents are "karma" for Russia's invasion.

"If something is launched into other countries' airspace, sooner or later unknown flying objects will return to (their) departure point," Ukrainian presidential adviser Mykhailo Podolyak wrote on Twitter on Monday.

MOLDOVA HIT

Moscow has been hitting Ukraine's energy infrastructure roughly weekly since early October as it has been forced to retreat on some battlefronts. This time, police in neighbouring Moldova were reported to have found missile fragments on its soil.

In Ukraine's southern Zaporizhzhia region, at least two people were killed and several houses were destroyed, the deputy head of the presidential office, Kyrylo Tymoshenko, said.

Air strikes also hit energy facilities in the regions of Kyiv and Vinnytsia in central Ukraine, Odesa in the south and Sumy in the north, officials said.

Forty percent of the Kyiv region was without electricity, regional governor Oleksiy Kuleba said, praising the work of Ukrainian air defences for what he said appeared to be a lack of "critical consequences" of Monday's attacks.

Air defences shot down most of the Russian missiles fired on Monday and energy workers had already begun work on restoring power supplies, Ukrainian President Volodymyr Zelenskiy said.

Ukraine had only just returned to scheduled power outages from Monday rather than the emergency blackouts it has suffered since widespread Russian strikes on Nov. 23, the worst of the attacks on energy infrastructure that began in early October.

Russia has said the attacks are designed to degrade Ukraine's military. Ukraine says they are clearly aimed at civilians and thus constitute a war crime.

A $60 per barrel price cap on Russian seaborne crude oil came into force on Monday, the latest Western measure to punish Moscow over its invasion. Russia is the world's second-largest oil exporter.

The agreement allows Russian oil to be shipped to third-party countries using tankers from G7 and EU countries, insurance companies and credit institutions, only if the cargo is bought at or below the $60 per barrel cap.

Moscow has said it will not abide by the measure even if it has to cut production. Ukraine wants the cap set lower: Zelenskiy said $60 was not low enough to stop Russia's assault.

 READ MORE

• War will be 'at reduced tempo' for months, says US
• Russia won't stop strikes until it runs out of missiles, President Volodymyr Zelenskiy says
• Kremlin says Kyiv can 'end suffering' by meeting Moscow's demands
• European Parliament declares Russia a state sponsor of terrorism

A Russian oil blend was selling for around $79 a barrel in Asian markets on Monday - almost a third higher than the price cap, according to Refinitiv data and estimates from industry sources.

'TRAGIC NIGHT'

Both sides on the ground in Ukraine reported casualties from other attacks overnight, on an industrial enterprise and another location in southern Ukraine, and on state-run accommodation in Russian-held territory in the east.

Three dormitories of the Donbas State Institute in the eastern Luhansk region were hit early on Monday by Ukrainian fire, killing nine people, the region's Russian-installed governor and Russia's state-run news agency TASS said.

Pro-Russian Governor Leonid Pasechnik said the dormitories were being used to house refugees and construction workers.

In Kryvyi Rih, among the largest cities in southern Ukraine, Russian rockets killed one person and wounded three just after midnight, the governor of the Dnipropetrovsk region, Valentyn Reznichenko, said.

"They aimed at an industrial enterprise," Reznichenko said on the Telegram messaging app without giving details.

"It was a tragic night," he said, while Oleksandr Vilkul, head of the Kryvyi Rih Military Administration, said: "The destruction is very significant."

In the southern Kherson region, where Ukraine forced Russian forces to withdraw across the Dnipro River last month, governor Yaroslav Yanushevych said further Russian shelling had killed two people in the past 24 hours.

Reuters could not independently verify the fighting reports.