Swedish intelligence agency Säpo is investigating a far-right attack plot on the Swedish parliament and prime minister's residence.

"I can confirm the fact that we have some information about alleged plots on the parliament and prime minister's residence," Säpo spokesman Patrik Peter told AFP.

"We have been informed about this for a while," Peter said, without giving details on when or how the intelligence service learned of the threat.

"We are now doing our best to get more information about it," he added.

Swedish tabloid Aftonbladet said Thursday an extremist group was plotting a politically-motivated attack against the Swedish parliament, the foreign ministry, and the prime minister's residence.

Citing unidentified sources, the tabloid said the attack was meant to be financed by the sale of the now-recovered Nazi German "Arbeit Macht Frei" sign. "I have no information to give you about that," the intelligence agency's spokesman said.

Swedish police on Wednesday told AFP they had no information about an alleged Swedish link to the sign's theft and had not been contacted by Polish authorities on the matter.

Polish investigators said Tuesday a foreign resident was the mastermind of the theft, but refused to confirm Polish news channel TVN24's report that the trail led to Sweden.