OSLO — Police on Wednesday vowed to get tougher in their questioning of the self-confessed perpetrator of last month's devastating attacks in Norway, in particular over whether he had any accomplices.

Questioning will happen this week, police prosecutor Paal-Fredrik Hjort Kraby told reporters.

"We can say that what will be central to that interrogation is that it will be more confrontational," he added.

Norwegian daily VG published a picture on its website of a van it said had transported rightwing extremist Anders Behring Breivik to Oslo police headquarters at around 9:00 am (0700 GMT) for interrogation.

Hjort Kraby would not confirm the report. They would only confirm the interrogation after it had taken place, he said.

Investigators would push especially hard for information about any possible accomplices, he added, even if it appeared that the 32-year-old killer had acted alone.

"That's the main issue, because if we find someone else, the investigation changes completely. But he's been very specific that he (acted) all alone," Hjort Kraby told AFP.

Behring Breivik, who claims to be on a "crusade" against a "Muslim invasion" of Europe, "could" have acted alone, he said.

"But he bought a lot of equipment, there was this farm he rented. He rented a lot of cars," he added. "These are questions we're looking into now."

Behring Breivik had been willing to talk and police expected to hold "lots" of interrogations with him from now on, Hjort Kraby told AFP.

"He's been more than willing to explain and give details, and as long as he stays like that we see a lot of interviews in the next weeks and months," he said.

On July 22, Behring Breivik set off a bomb in Oslo's government quarter, which killed eight people.

Hours later he killed 69 people, many of them teenagers, in a shooting rampage on the nearby island of Utoeya, where the ruling Labour party's youth wing was holding a summer camp.

He has so far been interrogated twice since his arrest on the island: the first, a week after the attacks, lasted seven hours.

During his second interrogation, held last Friday for 10 hours, police focused on going through his confession and his account of events.

The Oslo police has set up a special team to investigate Behring Breivik's crimes.

Deputy police commissioner Hans Halvorsen has said that around a hundred Oslo police officers were working on the case, with the help of colleagues from around the country.

Norwegian media have reported that the gunman called police himself from Utoeya, but Hjort Kraby would not say if his phone had been found.

"That's part of things we are looking into. But I can't confirm that, we're looking into it," he told AFP.

According to VG, Behring Breivik called the police emergency number 112 at the site of the massacre.

"Breivik. Commander. Involved in the anti-communist resistance against Islamisation. Mission accomplished and I will surrender to the Delta force:" he said in a three-second call, the paper said.

Officers from the elite Delta force arrested him soon after.

Since his arrest, Behring Breivik has made a series of outlandish demands, from wanting to be named head of the army to calling for the king to be forced to abdicate.

Before his only court appearance to date, a closed hearing on July 25, his requests to appear in uniform and to speak English were denied.

In a 1,500-page manifesto posted online before the attacks, Behring Breivik detailed his hatred of Islam, democracy and Marxism.

The youth wing of Norway's rightwing populist Progress Party (FrP), of which Anders Behring Breivik was once a member, announced Wednesday it would increase security for its summer retreat.

"Security is at the heart of our concerns," Ove Vanebo, the head of its youth wing, told AFP.

Some 170 people are expected for the summer retreat, which takes place over three days in Lillesand in southern Norway, he said.