Two bridges have collapsed in different Russian regions bordering Ukraine, derailing trains and killing at least seven people and injuring dozens, Russian authorities say.

The first bridge, in the Bryansk region on the border with Ukraine, collapsed on top of a passenger train on Saturday, causing the casualties. The train's driver was among those killed, state-run Russian Railways said.

Hours later, officials said a second train derailed when the bridge beneath it collapsed in the nearby Kursk region, which also borders Ukraine.

In that collapse, a freight train was thrown off its rails onto the road below as an explosion collapsed the bridge, local acting Governor Alexander Khinshtein said. The crash sparked a fire, but there were no casualties, he said.

Russia’s Investigative Committee, the country’s top criminal investigation agency, said in a statement that explosions had caused the two bridges to collapse, but did not give further details. Several hours later, it edited the statement, which was posted on social media, to remove the words “explosions” but did not provide an explanation.

The committee said that it would be investigating the incidents as potential acts of terrorism.

Bryansk regional Governor Alexander Bogomaz announced three days of mourning for the victims, starting Monday.

"The bridge was blown up while the Klimovo-Moscow train was passing through with 388 passengers on board," Bogomaz told Russia's public broadcaster.

Rescue workers cleared debris from both sites, while some of those injured were transported to Moscow for treatment.

Photos posted by government agencies in Bryansk appeared to show train carriages ripped apart and lying amid fallen concrete from the collapsed bridge. Other footage on social media was apparently taken from inside vehicles on the road that had managed to avoid driving onto the bridge before it collapsed.

Passengers help each other from the train. – AP 

Damage to railway tracks was also found Sunday by inspectors working on the line elsewhere in the Bryansk region, Moscow Railway said in a statement. It did not say whether the damage was linked to the collapsed bridges.

The areas in Russia's south have been subject to frequent attacks by Ukraine during the war that Russia started with its full-scale invasion more than three years ago.

Bryansk Governor Alexander Bogomaz said on Telegram that 47 people were hospitalised. Three children were among those injured with one in serious condition, he said.

The collapse in the Kursk region occurred early on Sunday while a freight train was crossing the bridge, Khinshtein said.

"Part of the train fell onto a road underneath the bridge," Khinshtein said.

He added that the locomotive caught fire, which was quickly extinguished. One of the drivers sustained leg injuries, and he and the team operating the train were taken to a local hospital, Khinshtein added.

Hours after the first collapse, a second bridge collapse was reported in the Kursk region, which also borders Ukraine. – X

He posted a photo of derailed carriages on a damaged bridge over a road.

Andrei Klishas, a senior member of the Federation Council, Russia's upper chamber of parliament, said on the Telegram messaging app that the incident in Bryansk shows that "Ukraine has long lost the attributes of a state and has turned into a terrorist enclave".

Russia's Baza Telegram channel, which often publishes information from sources in the security services and law enforcement, reported, without providing evidence, that according to preliminary information, the Bryansk bridge had been blown up.

Prominent Russian military blogger Semyon Pegov, who uses the name War Gonzo, called the Bryansk collapse "sabotage".

In a statement Sunday, Ukraine's military intelligence, known by the Ukrainian abbreviation GUR, said a Russian military freight train carrying food and fuel had been blown up on its way to Crimea. It did not claim the attack was carried out by GUR or mention the bridge collapses. The statement said Moscow's key "artery" with the Russian-occupied Zaporizhzhia region and Crimea has been destroyed.

Since the war began in February 2022, there have been continued cross-border shelling, drone strikes and covert raids from Ukraine into the Bryansk, Kursk and Belgorod regions that border Ukraine.

Russia's Ministry of Emergency Situations said on Telegram that efforts to find and rescue victims in the Bryansk incident continued throughout the night, and that some 180 personnel were involved in the operation.

A freight train was derailed in Kursk after a reported bridge explosion. – AP

Social media pictures and videos showed passengers trying to help others climb out of the Bryansk train's damaged carriages in the dark and firefighters looking for ways to reach passengers.

The train was going from the town of Klimovo to Moscow, Russian Railways said. It collided with the collapsed bridge in the area of a federal highway in the Vygonichskyi district of the Bryansk region, Bogomaz said. The district lies some 100km (60 miles) from the border with Ukraine.

US President Donald Trump has urged Moscow and Kyiv to work together on a deal to end the war, and Russia has proposed a second round of face-to-face talks with Ukrainian officials in Istanbul on Monday.

Ukraine has not committed to attending the talks, saying it first needed to see Russia's proposals, while a leading US senator warned Moscow it would be "hit hard" by new US sanctions.