Polish Prime Minister Donald Tusk said Tuesday that investigators identified two suspects allegedly responsible for an explosion on a railway line. 

According to Tusk, the two Ukrainian citizens and had been allegedly commissioned by Russian intelligence agents to carry out the blast before fleeing to Belarus. 

The Polish prime minister had earlier described the blast as an "unprecedented sabotage." A spokesperson from his government also said all evidence pointed to Moscow as having ordered the blast. 

What we know about the suspects

Tusk told the lower house of parliament that the duo had been collaborating with Russian secret services for a long time.

One of them is believed to have been previously convicted for "acts of sabotage" by a court in the western Ukrainian city of Lviv, Tusk said. The other one is a resident of Donbas, a Russian-occupied region in Ukraine.

Polish authorities are aware of their names but will not reveal them to the public because of ongoing investigations, the prime minister said. 

Poland on alert after railway blast

Top Polish military and intelligence officials were meeting on Tuesday after Tusk declared the explosion an act of sabotage. 

The Defense Ministry said army patrols were sent to check the safety of railways and other key infrastructure in the country's east.

Tusk, speaking to lawmakers on Tuesday, said Poland would introduce a higher threat level on certain railway lines.

"The head of the Internal Security Agency and the interior minister have requested that I introduce the third threat level, Charlie, due to terrorist threats," Tusk told parliament.

"This level will apply to certain railway lines, while the rest of the country will remain under the second alert level." 

What we know about the Polish railway sabotage

Two incidents on Sunday and Monday were reported. 

An explosion damaged railway tracks near the village of Mika, around 100 kilometers (60 miles) southeast of Warsaw.

Separately, power lines were destroyed in the area of Pulawy, about 50 kilometers (30 miles) from Lublin, in what Tusk said was "likely intended to derail a train." 

The railway route connects the Polish capital, Warsaw, to the Ukrainian border, and is considered vital for aid deliveries to Ukraine. 

Tusk referred to the disruptions as "perhaps the most serious national security situation in Poland since the outbreak of the full-scale war in Ukraine."

Edited by: Wesley Rahn