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."

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.

Investigators are see

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