Poland’s Prime Minister Donald Tusk said Monday that an explosion on a section of railway line used for deliveries to Ukraine was an “unprecedented act of sabotage.”

A Polish security source told The Associated Press that authorities are investigating whether the blast on Sunday on the line linking Warsaw to southeastern Poland is connected to Russia, Belarus or their proxies.

Tusk vowed that Poland will catch the perpetrators, “whoever they are.”

While visiting the site Monday, the prime minister said the incident had taken place on a line that is vital for delivering aid and weapons to Ukraine. Polish officials said they were sure an explosive device had been detonated on a section of track between Warsaw and Lublin, and later discovered damage to overhead cables on the same line.

Both were likely sabotage, Polish officials said.

NATO Secretary General Mark Rutte said the alliance is in close contact with Polish officials and is awaiting the outcome of the investigation.

Western officials have accused Russia and its proxies of staging dozens of attacks and other incidents across Europe since the invasion of Ukraine three years ago. Moscow’s goal, Western officials say, is to undermine support for Ukraine, spark fear and divide European societies.

In September, Poland said that with NATO assistance, it shot down a handful of Russian drones that overshot Ukraine and entered its airspace.

The likelihood that the two incidents on the railway line happened “on the order of foreign services” was “very high” said Tomasz Siemoniak, Poland’s security services minister.

A train driver on the line reported problems with the track around 7:40 a.m. on Sunday. Further inspection determined there was damage to a section of track near the village of Mika, about 100 kilometres southeast of Warsaw, officials said.

“We can say beyond any doubt that an explosive device was detonated, damaging the railway tracks,” Polish Interior Minister Marcin Kierwiński said.

It wasn’t immediately clear whether the explosion and damage occurred late Saturday or early Sunday. Two passengers and several staff members were on the train, but no injuries were reported, officials said.

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Defence Minister Władysław Kosiniak-Kamysz said Monday that the army plans to examine 120 kilometres of the Warsaw-Lublin-Hrubieszów line, which links the Polish capital to Ukraine by rail and road.

Another train on the Świnoujście-Rzeszów route that travels along the same line was forced to stop Sunday night at Puławy, about 50 kilometres from Lublin, after the overhead electrical cables were damaged, Kierwiński said Monday.

Around 60 metres of power lines were damaged, he said. Puławy is situated between Mika and Lublin on the same train line linking Warsaw to the Ukrainian border.

At the time the train stopped, there were 475 passengers on board. No injuries were reported, despite the damaged power lines breaking several of the train’s windows.

Speaking with Rutte at a news conference in Brussels following the announcement from Poland, Finland’s President Alexander Stubb said European nations need to keep “cool heads.”

He warned about overreacting to incidents of alleged Russian disruption, warning that, “this is unfortunately the new normal. What Russia is trying to do is to destabilize our societies through information and other campaigns.”

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In an interview with Saturday at a Finnish military base, Stubb said Moscow is conducting “two types of warfare” — a kinetic war in Ukraine and a hybrid war in Europe.

“The line between war and peace has been blurred,” Stubb said.

Russia has repeatedly denied past allegations of sabotage in incidents involving drone reports near airport and severed underwater cables in European waters.