Polish prosecutors have initiated an investigation into "acts of sabotage of a terrorist nature" committed for the benefit of foreign intelligence.
Poland's Prime Minister Donald Tusk said on Tuesday that Russian intelligence services orchestrated an explosion on a railway line used to transport aid to Ukraine, carried out by two Ukrainian nationals working as their agents.
"The identified perpetrators are two Ukrainian citizens who have been cooperating with Russian intelligence services for a long time. Their identities are known," Tusk told lawmakers in the parliament in Warsaw.
The Polish premier did not disclose the names of the alleged perpetrators.
Earlier on Tuesday, a government spokesperson said evidence suggested that Russian intelligence services appeared to have ordered the sabotage over the weekend.
"Everything indicates" that the rail incident over the weekend was "initiated by the Russian secret services," Jacek Dobrzyński, the spokesperson for Poland's secret services minister, said in comments reported by the Polish Press Agency (PAP).
Dobrzyński spoke after a meeting of the governmental National Security Committee, which took place on Tuesday morning and included military commanders, heads of intelligence services and a representative of the president.
In what Tusk called on Monday an "unprecedented act of sabotage," a segment of a rail line linking Poland's capital Warsaw to the border with Ukraine was blown up over the weekend.
Another segment further south was also damaged in what officials say was also likely sabotage.
Polish prosecutors are investigating "acts of sabotage of a terrorist nature" directed against railway infrastructure and committed for the benefit of foreign intelligence.
"These actions brought about an immediate danger of a land traffic disaster, threatening the lives and health of many people and property on a large scale," prosecutors said in a statement.
Defence Minister Władysław Kosiniak-Kamysz told Radio Zet on Tuesday that authorities were investigating the planned use of a camera found near the damaged tracks on the Warsaw-Lublin route.
He also said army patrols have been sent to check the safety of railways and other key infrastructure in the east of the country.
In the first incident, an explosion damaged the tracks near the village of Mika, about 100 kilometres southeast of Warsaw and, in a separate incident, power lines were destroyed in the area of Puławy, 50 kilometres from Lublin.
Trains carrying passengers were forced to stop at both locations, but no one was hurt.
"The explosion was most likely intended to blow up the train," Tusk said on Monday in reference to the Mika incident.
The damage caused at both locations has since been repaired.