Norway has demanded an explanation from the United States after a television documentary said its embassy had conducted illegal surveillance of hundreds of Norwegian residents over the past decade.

According to the TV2 News channel, the US embassy in Oslo employed between 15 and 20 people, including former high-ranking police officers, to monitor local residents in a bid to ward off attacks on US interests in the country.

The surveillance had been going on since 2000, said the report.

Embassy-hired employees photographed people taking part in demonstrations and added their names and personal data to a special computer database, SIMAS (Security Incident Management Analysis System), TV2 reported.

If the report is correct, the embassy conduct would constitute a violation of Norwegian laws.

The Norwegian foreign ministry said it had held a meeting with the US embassy Wednesday to try to find out what had taken place.

The ministry had asked for information on whether Norway had ever been informed about the surveillance programme and what it involved, ministry spokeswoman Marte Lerberg Kopstad said in a statement.

"The meeting did not clarify these matters much. It is therefore important that we now get all the facts on the table," she added.

Speaking to reporters, Foreign Minister Jonas Gahr Stoere meanwhile refused to speculate on the accuracy of the TV2 report but said "if Norwegian laws were broken it is serious."

In Washington, State Department spokesman P.J. Crowley told TV2 that Norwegian authorities had been informed in advance about the surveillance activities.

Officials in the Scandinavian country have meanwhile insisted they knew nothing about what was going on.

The US embassy in Oslo meanwhile published two separate statements Thursday.

In the first, it stressed "we are prepared to work intensively to address any questions the Norwegian government might have on this or any other matter."

In a second statement, the embassy vowed to help the government get to the bottom of the "insinuations and allegations made on the Norwegian TV2 news feature last night."

"We work very closely with host country authorities to ensure the safety and security of US embassies and all our visitors around the world," it said.

The Norwegian justice ministry meanwhile said it had begun checking with all the country's security agencies that may possibly have been aware of the US surveillance programme.

The head of the Norwegian Data Inspectorate, which is tasked with protecting personal data and ensuring against violations of the right to privacy, said he believed the embassy's reported actions were illegal.

"We see this as a violation of the Personal Data Act," Bjoern Erik Thon told TV2.

"There are rules regarding what kind of personal information you can collect in Norway and how it should be stored and how long it can be stored. And this set of rules has been completely disregarded in this case," he added.