A man armed with a knife attacked clergy and worshippers at a Catholic monastery in southern Poland on Thursday, killing one person and wounding four others.

Police said the attacker, a 31-year-old Ukrainian national, carried out the stabbing at the 17th-century Benedictine abbey in Jaroslaw, a town located approximately 30 kilometres from the Ukrainian border.

The suspect had entered Poland from Germany earlier that day. According to police, he took a knife from the abbey's kitchen before attacking four men and one woman, including a priest who was in a confessional at the time.

"Three of the victims are clergy members and two are laypeople; one person has died," said prosecutor Bozena Harasz-Wołoszyn. Two other victims were described as being in critical, life-threatening condition.

Polish media identified the person killed as a priest. Police did not immediately confirm the identity of the deceased.

Prosecutors said the attacker had been heading toward a nearby border crossing to re-enter Ukraine but instead went to the abbey. The motive for the attack remains unclear.

Harasz-Wołoszyn said doctors would assess whether the suspect had been under the influence of alcohol at the time of the attack.

Polish Prime Minister Donald Tusk said the "murderer" would be "punished with the full severity of Polish law."

Ukrainian President Volodymyr Zelenskyy condemned the attack, calling it a "horrific crime." He added: "The perpetrator of this evil must bear full responsibility in accordance with the law. We are shaken by this evil. Justice must be done."

The mayor of Jaroslaw, Marcin Nazarewicz, declared a period of mourning until the end of the week.

Stabbings, particularly at religious sites, are extremely rare in Poland, one of Europe's most Catholic countries.