The suspects have left Poland and fled to Belarus, a close ally of Moscow, Tusk said.
He made the statement after meeting behind closed doors with military commanders, security officials and a representative of the president to discuss the case and broader national security risks.
Prime Minister Donald Tusk speaks in parliament on Tuesday. Photo: PAP/Tomasz Gzell
Polish officials have vowed to bring to justice those responsible for detonating an explosive device on a key railway line linking Warsaw with the eastern city of Lublin, a strategic route for transporting Western aid to Ukraine.
The blast occurred over the weekend near the village of Mika, about 100 kilometres southeast of Warsaw, according to reports.
Tusk said on Monday that the explosion destroyed a section of track between Warsaw and Lublin in what he called an "unprecedented act of sabotage" against Poland's security.
In another incident on the same line, a train carrying 475 passengers was forced to make an emergency stop near the town of Puławy on Sunday after encountering track damage, officials told reporters.
No one was injured, and the government has launched a wide-ranging investigation.
Tusk told lawmakers on Tuesday that Poland was "in constant contact with the security services of allied countries" as it works to track down the suspects.
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Source: TVP Info, IAR, PAP