Two Ukrainians working for Moscow have been identified as suspects in two cases of rail line sabotage in Poland, Prime Minister Donald Tusk said during a speech in parliament this afternoon.
The pair "have been operating and cooperating with Russian services for a long time", Mr Tusk said, citing information from prosecutors and investigating authorities.
He added that authorities are aware of the suspects' identities, but that they would not be made public pending further investigation.
The two acts, which occurred between Saturday and yesterday, damaged a railway line helping to supply close ally Ukraine.
Mr 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".
The first incident involved a steel clamp, which was attached to a track along the line and "likely intended to derail a train", said Mr Tusk.
The second was the detonation of a military-grade explosive device during the passage of a freight train along the track.
No one was injured during either incident.
One of the perpetrators is believed to be a Ukrainian citizen who in May was convicted for "acts of sabotage" by a court in Lviv, the prime minister said.
The other is a resident of Donbas, a Russian-occupied region in Ukraine, who along with the first crossed into Poland from Belarus this autumn.
Shortly after the incidents, the two suspects are believed to have left Polish territory for Belarus.
In total, 55 individuals have been detained, and 23 arrested regarding acts of sabotage in Poland, said Mr Tusk.
Poland - a NATO and EU member - has become the main hub for transporting military and humanitarian aid to its war-affected neighbour.
Several European leaders, including EU chief Ursula von der Leyen, NATO Secretary General Mark Rutte, and Ukrainian Foreign Minister Andriy Sybiga, expressed their solidarity with Poland on Monday following the announcement of the incidents.
Mr Sybiga on Monday suggested that their purpose could have been "to test responses".
Mr Tusk claimed that the purpose of the attack also involved its "social and political consequences," including the buildup of anti-Ukrainian sentiments.
He called this "particularly dangerous in countries like Poland".
Following Mr Tusk's speech, the Kremlin accused Poland of "Russophobia".
"It would be strange if Russia wasn't blamed straight away," Kremlin spokesman Dmitry Peskov told state TV, without outright denying the accusation that Moscow was involved and adding: "Russophobia is, of course, flourishing there."
In recent years, Poland has restricted the movement of Russian diplomats on its soil, ordered the closure of two Russian consulates, and detained a total of 55 individuals suspected of acting on behalf of Moscow.
According to Mr Tusk, the names of the suspects may be disclosed later.
