Ukrainian forces have blown up the main railway connecting Russia to China, in one of the most ambitious strikes deep into enemy territory.

Operatives from Kyiv’s SBU security services blew up a freight train in the Severomuysky Tunnel on the Baikal Amur mainline in Buryatia, more than 3,000 miles east of Ukraine.

Security sources said four explosions were triggered as the train passed through the tunnel, which sits north of Mongolia.

“This is the only serious railway connection between the Russian Federation and China,” a Ukrainian military official said.

“Currently, the Russians use this route, including for military supplies. After the explosion, it was paralysed.”

Moscow has launched a criminal investigation into the incident, which was described as a “terrorist attack”.

Ukraine has not publicly claimed responsibility for the apparent strike.

There were also unconfirmed reports that a second explosion had targeted the railway line linking Russia and China on Friday.

Footage of the purported second strike circulated on social media appeared to show flames bursting out from the top of a cargo train.

The fire appeared to spread as burning liquids rolled down a hill towards the camera from the stricken train.

Russian trains have now been diverted over the 35-metre-tall Devil’s Bridge.

Some local reports suggested explosives were planted on the bridge for the second attack.

The Telegraph could not immediately verify the claims.

‘Our people are everywhere’

“Russian special services should get used to the fact that our people are everywhere,” a source told the Ukrainska Pravda outlet. “Even in distant Buryatia.”

A second source told the Kyiv Post that Russian workmen were attempting to make the tunnel operational again.

The extent of the damage to the tunnel was not immediately clear.

Russia’s RZHD state railroad firm said: “While driving through the tunnel, the locomotive crew of the cargo train noticed smoke from one of the diesel fuel tanks.

“The train was stopped, and two fire extinguishing trains were sent from nearby towns to help.

“The movement of trains was not interrupted, it was organised along a bypass section with a slight increase in travel time.”

Baza, a pro-Kremlin outlet on the Telegram messaging app, also reported that the fuel tank on the train had caught fire. “What caused the fire is still unknown,” it added.

The East Siberian Transport Prosecutor’s Office said there had been no casualties.

Russia “will take response measures based on the results of the inspection, in case there are grounds to do so”, the prosecutors’ office added.

“Train traffic has not been interrupted along the route, it was organised on a detour route with a slight increase in travel time,” the Russian railway provider said in a statement.

The apparent strikes are a sign of Ukraine’s growing willingness to strike deep inside Russian territory to disrupt its occupation.

Kyiv rarely claims responsibility for such strikes, but intelligence officials often take credit anonymously for their achievements.

Counter-offensive ‘didn’t achieve desired results’

It came as Volodymyr Zelensky, the president of Ukraine, conceded the country’s counter-offensive “didn’t achieve desired results”.

He blamed the slow progress on Kyiv not receiving all the weapons it needed from allies and the small size of his fighting force, adding: “This does not mean that we should give up; that we have to surrender.”

Mr Zelensky also warned that defences must be quickly bolstered across all parts of the front line, after meeting with commanders in areas where Russia is exerting pressure in the south and east.

Meanwhile, Russia’s FSB security service said on Friday it had detained a Russian-Italian dual citizen accused of attacking a military base in the west of the country with exploding drones.

The man, who allegedly was recruited by Ukrainian intelligence officers in Turkey and underwent training in Latvia, was also accused of derailing a cargo train in November.