Air alerts sounded across Ukraine on Monday and officials urged civilians to take shelter from what they said looked like a large wave of strikes, the latest in relentless rounds of air attacks by Russia since its February 24 invasion.

It followed reported incidents at two air bases inside Russia overnight, both hundreds of kilometres from Ukraine.

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

Roman Busargin, regional governor of Saratov, further to the southeast, acknowledged separate reports of a blast at a base housing bomber planes that are part of Russia's strategic nuclear forces.

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.

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

Ukraine did not claim responsibility for either incident, or for earlier mysterious explosions.

Weapons stores and fuel depots have been hit in Russian regions close to the border with Ukraine and at least seven warplanes destroyed in Crimea, annexed by Russia from Ukraine in 2014.

Ukraine has said 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.

The Russian attacks later on Monday killed two people in the Zaporizhzhia region where several houses were destroyed, the deputy head of the presidential office, Kyrylo Tymoshenko, said in one of the first reports of the damage.

Buildings had been hit in the suburbs of the city of Zaporizhzhia and some Russian missiles had been shot down, a city official said.

The governor of the Kyiv capital region said its air defences were working there, and told residents to remain in shelters.

An energy provider said power had been knocked out in the northern region of Sumy by the latest missile strikes.

Meanwhile, Putin on Monday drove a Mercedes across the Crimean Bridge linking southern Russia to the annexed Crimean peninsula, less than two months since an explosion tore through one of the Kremlin chief's showcase infrastructure projects.

The 19km road and rail bridge, which was opened by Putin in 2018, was bombed on October 8 in an attack Russia said was carried out by Ukraine.

Putin, accompanied by Deputy Prime Minister Marat Khusnullin, was shown on state television behind the wheel of a Mercedes, asking questions about where the attack took place.

"We are driving on the right hand side," Putin said, as he drove across the bridge.

"The left side of the bridge, as I understand it, is in working condition, but nevertheless it needs to be completed. It still suffered a little, we need to bring it to an ideal state."

Putin also walked along parts of the bridge, Europe's largest, to inspect sections that are still visibly scorched.

Ukraine never claimed responsibility for the bombing of the bridge on the morning of October 8, a day after Putin's 70th birthday.

Russia's Federal Security Service said the attack was organised by Ukrainian military intelligence.

The explosion wrecked one section of the road bridge, temporarily halting traffic across the Kerch Strait.

The blast also destroyed several fuel tankers on a train heading towards the annexed Crimean peninsula from neighbouring southern Russia.

with AP

Australian Associated Press