Authorities in Norway say they have three suspected al Qaeda members in custody.
The three men were arrested on suspicion of "preparing terror activities." They had been under surveillance by law enforcement officials for more than a year. Their names were not disclosed.
Norweigan and U.S. officials said it was not clear the men had selected a target for the attacks, but believe the men were planning attacks with portable peroxide bombs, just like the ones at the heart of last year's thwarted attack in the New York City subway.
U.S. Attorney General Eric Holder called the New York subway bombing plot one of the most serious since the attacks of September 11, 2001.
Janne Kristiansen, head of Norway's Police Security Service, said one of the men was a 39-year-old Norwegian of Uighur origin, who had lived in Norway since 1999. The other suspects included a 37-year-old Iraqi and a 31-year-old citizen of Uzbekistan, both of whom have permanent residency permits in Norway.
Al Qaeda's second in command, Ayman al-Zawahri, has called for attacks on Norway among other countries.
Magnus Norell, a terrorism expert at the Swedish Defense Research Agency, told the Associated Press that Norway's 500 troops in Afghanistan could be a factor, as could the 2006 controversy sparked by a Danish newspaper's publication of 12 cartoons depicting the Prophet Muhammad.
Norell said the controversy has extended to neighboring Norway and Sweden after newspapers there republished the cartoons and later published similar cartoons. Images of Muhammad, even favorable ones, are considered blasphemous by many Muslims.