Poland’s police said on Sunday that they had received information about a possible plot to attack the country’s prime minister, Donald Tusk, and the defence minister, Władysław Kosiniak-Kamysz.

Poland has been on high alert after a series of school stabbings this week and fears of a possible Russian hybrid attack to weaken western support for Ukraine in its war against Moscow’s invasion.

“We take this matter extremely seriously,” said Jacek Dobrzyński, a spokesperson for the minister overseeing Poland’s intelligence services. He said authorities considered there was a high probability the threats would have been carried out.

Dobrzyński was speaking after an emergency meeting of security services, police and interior ministry officials on Sunday.

He said information about the threat first reached the state protection service, which passed it to the police but gave no further details.

Adam Szłapka, a spokesperson for the Polish government, said Tusk had learned about the “exceptionally concerning” threat first thing on Sunday morning.

Dobrzyński said the Polish services were receiving an increasing number of threats of various kinds every month.

Three attacks at schools in Mazovia province in central Poland took place within four days this week, leaving one person dead and many wounded, including children.

Tusk said last month that Russia was likely to intensify hybrid attacks by Russia on Nato’s eastern flank.

He said Moscow was planning strikes with drones or rockets on European countries that back Ukraine, among them Poland, with the goal of weakening western support. Russia has denied any such plans.