Quoting unnamed military sources, NRK said that the bomb discovered at the farm Anders Behring Breivik had rented, some 140 kilometers (85 miles) north of Oslo, weighed between 1.0 and 1.5 tons and was ready to use.

In comparison, the car-bomb he set off outside government offices in Oslo, killing eight people, weighed 950 kilograms (2,095 pounds), according to expert estimates.

Without giving any details on the size of the bomb, Norwegian police denied that they had found any explosive device that was "ready to deploy,” contradicting a key element of the television report.

Oslo police spokesman Roar Hanssen said on Twitter that: "A lot of modified fertiliser was found at the farm."

"However that is just one of many bomb components. Important elements were missing and it still needed a lot of work before that became a bomb able to be deployed," he said.

On July 27, police said they had found and destroyed explosives stored at the isolated farm, but did not provide any details on the quantity or what condition they were in when they were discovered.

Behring Breivik, a 32-year-old rightwing extremist who had been planning the attacks for years, had rented the farm several months before the July 22 carnage, officially to grow vegetables but most likely to have a reason to purchase chemical fertiliser without arousing suspicion.

According to a Norwegian agricultural cooperative, Behring Breivik received a delivery at the beginning of May of six tons of fertiliser, which may have been used to make the bombs.

After setting off his bomb in Oslo, the killer went on a shooting rampage on the nearby island of Utoeya, where the ruling Labor Party's youth wing was hosting a summer camp, killing 69 more people, many of them teenagers.

According to police, Behring Breivik has said during interrogations that he had "other targets" in mind that day, and media has reported that he had planned to attack the royal palace and the Labour Party headquarters.

Currently being held in preventive custody in a high-security prison near Oslo, Behring Breivik has confessed to the attacks and claims he acted alone.