Explosions rocked two air bases in Russia on Monday, Russian media reported.
One of the blasts reportedly happened at a base which houses nuclear-capable strategic bombers that have been involved in launching strikes against Ukraine.
Neither Ukrainian nor Russian authorities immediately commented on the possible cause of the explosions.
Russian state news agency RIA Novosti said three servicemen were killed and six others injured when a fuel truck exploded at an air base in Ryazan in western Russia early on Monday.
A plane was also damaged in the blast at the base, which houses long-range flight tankers that serve to refuel bombers in the air.
Separately, authorities in the Saratov region along the Volga River said they are checking reports of an explosion in the area of the Engels air base, which houses Tu-95 and Tu-160 strategic bombers that have been involved in launching strikes on Ukraine. Those bombers are capable of carrying nuclear weapons.
Saratov regional governor Roman Busargin said there was no damage to civilian facilities and added that the authorities are checking whether there have been any incidents at military facilities.
Regional media reported sounds of a powerful explosion near the Engels base, and some residents were quoted as saying they saw a flash of light coming from the area.
Asked whether Russian President Vladimir Putin has been briefed about the Engels base explosion, Kremlin spokesman Dmitry Peskov said simply that the president is being regularly informed about ongoing developments.
In Ukraine on Monday, the office of President Volodymyr Zelensky said three rocket strikes hit his hometown of Kryvyi Rih in south-central Ukraine, killing a factory worker and injuring three other people.
In the north-eastern region of Kharkiv, one person was killed in strikes by S-300 missiles on civilian infrastructure in the town of Kupyansk, it said.
The war that began with Russia’s invasion of Ukraine on February 24 has displaced millions from their homes, killed and injured an untold number of civilians, and shaken the world economy – notably through the fallout on the prices and availability of foodstuffs, fertiliser and fuel that are key exports from Ukraine and Russia.
Western countries on Monday began imposing a 60-dollars-per-barrel price cap and a ban on some types of Russian oil, part of new measures aimed at stepping up pressure against Moscow over the war.
The move has prompted a rejection from the Kremlin and also criticism from Mr Zelensky – whose government wants the cap to be half as high.
Russian Deputy Prime Minister Alexander Novak, who is in charge of energy issues, warned in televised comments on Sunday that Russia will not sell its oil to countries that try to apply the price cap.
“We will only sell oil and oil products to the countries that will work with us on market terms, even if we have to reduce output to some extent,” he said in televised remarks hours before the price cap came into effect.
The 27-country European bloc also imposed an embargo on Russian oil shipped by sea.
Russia, the world’s second biggest oil producer, relies on the sale of oil and gas to underpin its economy, which has already come under sweeping international sanctions over Mr Putin’s war in Ukraine.
In recent weeks, Russia has been pounding Ukrainian infrastructure – including power plants – with military strikes and keeping an offensive going in the east, notably in and around the town of Bakhmut.
Russian forces have also been digging in near the southern city of Kherson, which was recaptured by Ukrainian forces last month after an eight-month occupation.