Polish Prime Minister Donald Tusk has said that an explosion which damaged a railway line helping to supply close ally Ukraine was an "unprecedented act of sabotage".
The damage, which authorities have said was discovered yesterday, directly targeted "the security of the Polish state and its civilians", Mr Tusk wrote in a post on X.
Poland's National Prosecutor's Office said it had launched a probe into what it described as "sabotage of a terrorist nature" that it alleged was committed "on behalf of a foreign intelligence service".
Journalists at the scene of the incident in Mika, some 100 kilometres southeast of Warsaw, found that the tracks were already being repaired.
The area was cordoned off and being guarded by police and railway security guards.
Poland - a NATO and EU member - has become the main hub for transporting military and humanitarian support to neighbouring Ukraine since Russia's full-scale military invasion in February 2022.
Poland blames Russia for what it says are multiple incidents of sabotage since then, including drone incursions and arson attacks, which the Kremlin has angrily denied.
In recent years Poland has restricted the movements 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.
Mr Tusk visited Mika and called the rail route, which is used daily by dozens of trains, "critically important", including for delivering aid.
The damage to the track "was likely intended to derail a train", he said, but the danger was averted as a driver spotted the damage.
No one was injured.
Interior Minister Marcin Kierwinski said the explosion was triggered via a cable, a fragment of which was found at the scene.
"We will catch the perpetrators, whoever they are," said Mr Tusk in his X post.
He later said the country's national security committee, including military commanders and a representative from the presidency, would meet Tuesday.
NATO Secretary General Mark Rutte stated in Brussels that the alliance remains "in close contact with Polish authorities".
"The threats to our security are real and growing," EU chief Ursula von der Leyen said on X in response to Tusk's post, calling on Europe to "urgently boost capacity to protect our skies and our infrastructure".
Meanwhile, Ukrainian Foreign Minister Andriy Sybiga expressed his "solidarity with friendly Poland" and promised Ukrainian assistance in the ongoing investigation "if called upon".
He suggested the incident could be "another hybrid attack by Russia - to test responses".
Romanian villages evacuated after Russian drone attack in south Ukraine
It comes as Romanian authorities said they had evacuated two villages near the Ukrainian border, after a Russian drone attack in southern Ukraine set fire to a ship carrying liquefied petroleum gas.
Since invading Ukraine in 2022, Moscow has repeatedly attacked Ukrainian ports in the Danube region, triggering alerts in neighbouring Romania.
Following a nightly drone attack on a Danube river port in southern Ukraine, a ship carrying LPG had caught fire, Romanian authorities said.
Due to the ship's "proximity to Romanian territory and the nature of its cargo", authorities ordered the evacuation of the village of Plauru, situated across the Danube from the Ukrainian port city of Izmail.
Later, the authorities also ordered the evacuation of the nearby village of Ceatalchioi, which has just under 300 inhabitants. Road and naval traffic in the area has been halted.
The evacuations were the first ever in Romania as a result of the war in Ukraine.
Prior to the evacuation, residents in Plauru received alerts on their mobile phones, warning them of possible objects falling to the ground after the attack.
According to Romania's defence ministry, "no unauthorised incursions into national airspace were detected" after "Russian Federation forces attacked areas of Ukraine near the river border with Romania" overnight.
Macron, Zelensky in deal for Ukraine to buy fighter jets
Meanwhile, French President Emmanuel Macron and Ukrainian counterpart Volodymyr Zelensky have signed a letter of intent for Ukraine to acquire up to 100 Rafale fighter jets and other air defence hardware from French companies, the French presidency has said.
In a boost for Ukraine, the accord sets out possible future contracts for the acquisition by Ukraine of 100 Rafale fighter jets "with their associated weapons" as well as the new generation SAMP-T air defence system under development, radar systems and drones.
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The letter of intent, which is not a purchase and sales contract, is projected to be realised "over a timeframe of about 10 years", the French presidency added.
Mr Zelensky's visit to Paris is part of a brief tour of his western allies that saw him seal an energy deal with Greece yesterday and will include a visit to Spain tomorrow.
Mr Zelensky and Mr Macron started the day at the Villacoublay air base near Paris for a presentation of France's Rafale fighter jet.
They will also look at drones and France's SAMP-T surface-to-air missile system it developed with Italy.
The Ukrainian president last month signed a letter of intent that could see his country buy up to 150 Swedish Gripen fighters.
Ukraine already has one of the current SAMP-Ts that can counter fighter jets as well as cruise missiles and tactical ballistic missiles. A new-generation, more powerful SAMP-T system should become available from 2027.
France has already promised more of its Aster missiles that can be fired from the SAMP-T or from aircraft to Ukraine.
"We are also working on deliveries of other advanced air-defence systems and strengthening our aviation - a major deal with France is in preparation and will be finalised soon - a historic agreement," Mr Zelensky said in a video released on Friday.
Ukraine has said it plans to use more than 4.5 million drones this year, for frontline attacks and to fight off Russian drones.
Mr Zelensky's ninth visit to France since the Russian invasion in 2022 comes as his government battles a new corruption scandal, this time involving its energy firms.
France's Europe Minister Benjamin Haddad said in a television interview on Saturday that governments had to be "extraordinarily vigilant" about corruption as Ukraine presses its case to join the European Union.
