Polish investigators have identified two Ukrainian nationals suspected of carrying out an explosion on a strategically important railway line on behalf of Russia, Prime Minister Donald Tusk said on Tuesday.

Tusk, speaking to parliament, said both men had a long relationship with Russian intelligence.

The Kremlin rejected the accusations.

One suspect was convicted of sabotage by a court in Lviv, Ukraine in May and later travelled to Belarus, Tusk said.

The second is from Ukraine's eastern Donbass region, which Moscow and its separatist allies have been striving to conquer since 2014.

Both men are believed to have entered Poland from Belarus in the autumn and left the country via the Terespol border crossing shortly after the rail attack.

The tracks on the route from Warsaw to the eastern city of Lublin were destroyed at the weekend by an explosive device. The damage was spotted in time by a train driver and communicated to the control centre, which shut down the track.

No passengers or railway staff were injured.

For certain railway lines in Poland, the heightened alert level Charlie, which is used in NATO countries in the event of a confirmed threat, will be in effect immediately, Tusk said. The army will be responsible for guarding them.

Poland, a NATO and EU member and one of Ukraine's closest political and military allies, has feared sabotage attacks since Russia invaded its eastern neighbour.

The railway network is considered a target, as many Western military deliveries to Ukraine pass through Poland. The affected line leads to the border village of Dorohusk and onward into Ukraine.

A heightened NATO alert level, Charlie - triggered in the event of a confirmed threat - has been imposed to certain railway lines, which will be guarded by the arm, authorities said.

Tusk said investigators had established that the device attached to the rails detonated around 9 pm on Saturday as a freight train passed over it.

The explosion failed to derail the train and caused only minor damage to a wagon. "The driver did not even notice it as he passed," he said.

The charge was a military-grade C4 explosive triggered via a 300-metre-long electrical cable, Tusk said. Unexploded material was also recovered at the scene. He added that an earlier attempt to cause a derailment using a clamp fixed to the same line had failed.

Tusk said Russia aimed not only to disrupt infrastructure but also to create "disorganization, chaos, panic, speculation and uncertainty."

Recruiting Ukrainians for sabotage, he said, was intended to inflame anti-Ukrainian sentiment in Poland.

Tusk said that Belarus and Russia would be asked to extradite the two Ukrainian suspects. "But we will also take other measures that will hopefully lead to the swift arrest of the perpetrators and their accomplices," he said.

Last year, the Polish government blamed Russian intelligence for a large fire at a Warsaw shopping centre. In October, eight people were arrested on allegations of planning sabotage and espionage on Moscow's behalf.

The Kremlin rejected Tusk's accusations. "Russia is being held responsible for all manifestations of hybrid and direct warfare," Kremlin spokesman Dmitry Peskov told Russian state television.

In his view, it would be strange if Russia had not been blamed straight away this time. Poland is always at the forefront. "In this respect, Russophobia is naturally flourishing there in all its splendour," said Peskov.