Two Ukrainian citizens working for Russia are suspected of blowing up a railway line in Poland over the weekend, Polish Prime Minister Donald Tusk said Tuesday. On Wednesday, the row between the NATO member and Russia escalated, with Poland's top diplomat labeling the incident an "act of state terror."

Speaking to the Sejm, the lower house of the Polish parliament, Tusk said the two suspects had been collaborating with Russia's secret services for a long time. He said their identities were known but could not be revealed to the public because of ongoing investigations. The pair have already left Poland.

As NATO allies accuse Russia of hybrid warfare, U.S. joins war games in Sweden focused on disrupting sabotage
Tusk has described the explosion on a rail line linking Poland's capital, Warsaw, to the border with Ukraine as an "unprecedented act of sabotage."

That rail line is being used to transport aid to Ukraine, Polish officials said.

On Wednesday, Poland's foreign minister said he would order the closure of the last Russian consulate still operating in the country as part of the country's response to the sabotage.

Radek Sikorski said he had repeatedly warned Russia that its diplomatic and consular presence would be reduced further if it didn't cease hostile actions against Poland, Polish news agency PAP reported.

"In connection with this, though it will not be our full response, I have decided to withdraw consent for the operation of the last Russian consulate in Gdansk," he said. He added that Russia would be formally notified within hours.

"This time it was an act of not only subversion, as happened before, but an act of state terror as its clear intention was to cause human casualties," Sikorski said. The closure will leave Russia with only its embassy in Warsaw.

In response, Moscow will "reduce Poland's diplomatic and consular presence in Russia," the state news agency RIA Novosti cited Russian Foreign Ministry spokeswoman Maria Zakharova as saying.

In a separate incident, which Polish officials are also now confirming as sabotage, power lines over another segment of the same rail line further south were also damaged.

A meeting of the governmental National Security Committee had taken place earlier on Tuesday, with the participation of military commanders, heads of the intelligence services and a representative of the president.

Army patrols have been sent to check the safety of railways and other key infrastructure in the east of the country, the defense minister said.

Polish prosecutors have initiated an investigation into "acts of sabotage of a terrorist nature" directed against railway infrastructure and committed for the benefit of foreign intelligence.

"These actions brought about an immediate danger of a land traffic disaster, threatening the lives and health of many people and property on a large scale," prosecutors said in a statement.

In the first incident, an explosion damaged the tracks near the village of Mika, about 60 miles southeast of Warsaw and, in a separate incident, power lines were destroyed in the area of Pu?awy, about 30 miles from Lublin. Trains carrying passengers were forced to stop at both locations, but no one was hurt.

"The explosion was most likely intended to blow up the train," Tusk said on Monday in reference to the Mika incident.

The damage caused at both locations has been repaired.

Meanwhile, a rising number of mysterious drone sightings near airports and military bases has fueled concern in Europe over alleged Russian incursions into NATO airspace. Some of America's allies are saying the continent is in a gray zone between peace and war — accusing Moscow of escalating "hybrid warfare."

In September, Poland said that it scrambled warplanes to shoot down multiple Russian drones that entered its airspace during Moscow's latest attack on Ukraine, calling the incident an "act of aggression."

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