The Swedish Security Service, SAPO, declined to say whether the threat involved Islamic terrorists or if it was linked to aby Western intelligence officials to wage Mumbai-style shooting sprees or other attacks in Britain, France and Germany.
SAPO said the terrorism threat remained low in Sweden compared with other European countries, and that no attack was imminent.
"The assessment is based on intelligence indicating a shift in activities among certain groups in Sweden, judged to be targeted at Sweden," SAPO said in a statement.
Western security officials said Wednesday that a. Both European and U.S. officials said the plot was still in its early stages and not considered serious enough to raise the terror threat level.
A Pakistani intelligence official, who spoke on the condition of anonymity because he was not authorized to speak with the media, said eight Germans and two British brothers were at the heart of the al Qaeda-linked terror plot against European cities and had been calling acquaintances in Europe to plan logistics.
"I can't say whether (the Swedish alert level) has any links to other investigations because of our ongoing operation," SAPO spokesman Patrik Peter told The Associated Press. "But I want to underscore that there is no immediate threat."
SAPO said its chief Anders Danielsson made the decision to raise the alert level one notch to "elevated" in mid-September. That's the third level on a five-point scale introduced in 2008. Previously the threat level had remained steady at level two, or "low threat."
Peter declined to give details on what was behind the new assessment, but said it was not related to a far-right party entering Parliament for the first time in elections last week.
Magnus Norell, a terrorism expert at the Swedish Defense Research Agency, said he didn't know the details behind the assessment but "I don't think it's rocket science to guess that this is about Islamist groups."
"I would think that this means they are seeing more meeting and planning activity among those groups," Norell said. "But this is absolutely not the same thing as saying there are any concrete plans to attack."
On Friday, a U.S.-released study by counterterrorism experts said the U.S. and its European allies must work together better to stop radicalized Westerners who travel to terrorist training camps and return home to wage attacks.
Swedish security officials have warned that radicalized youngsters have traveled abroad to join Islamist insurgencies, primarily in Somalia.
"Right now there are about 10 (Swedes) in Somalia. But I want to underscore that this does not necessarily have any links" to the raised alert level in Sweden, Peter said.