The sources confirmed last week that building was under way at Ain Aouda, above a wooded gorge south of Rabat’s diplomatic district. Locals said they had often seen American vehicles with diplomatic plates in the area.

The construction of the new compound, run by the Direction de la Securité du Territoire (DST), the Moroccan secret police, adds to a substantial body of evidence that Morocco is one of America’s principal partners in the secret “rendition” programme in which the CIA flies prisoners to third countries for interrogation.

Amnesty International, Human Rights Watch and other groups critical of the policy have compiled dossiers detailing the detention and apparent torture of radical Islamists at the DST’s current headquarters, at Temara, near Rabat.

A recent inquiry into rendition by the Council of Europe, led by Dick Marty, the Swiss MP, highlighted a pattern of flights between Washington, Guantanamo Bay and Rabat’s military airport at Sale.

French intelligence and diplomatic sources said the most recent such flight was in the first week in December, when four suspects were seen being led blindfolded and handcuffed from a Boeing 737 at Sale and transferred into a fleet of American vehicles.

Morocco’s membership of a so-called “coalition of the willing” has led to tension within the kingdom, where Mohammed VI, 42, is trying to suppress a wave of Islamic fundamentalism, most powerfully expressed in the Casablanca bombings of May 2003, in which 12 suicide bombers — all of them Moroccan — killed more than 40 people.

More than 3,000 suspected radical Islamists have been arrested since, but some of the country’s higher-profile Al-Qaeda sympathisers have been released, including Abdallah Tabarak, a former bodyguard of Osama Bin Laden.

While much of the media is said to have been infiltrated by the DST, a few publications that dare to question official policy have accused the government of allowing Morocco to become “the CIA’s dustbin”.

Donald Rumsfeld, the American defence secretary — who described Morocco and Tunisia yesterday as “long-standing friends and constructive partners” in the fight against terrorism — is due to visit today. Among the topics expected to be discussed with officials is the opening of a new FBI office in Morocco.

Last Friday the country witnessed its first protests against the Danish cartoons of the prophet Muhammad. They were highly organised and controlled but created a sense of apprehension in the capital before Rumsfeld’s talks.

Morocco has an estimated 30,000 policemen for a population of 30m and many people seem scared of speaking to strangers. A Sunday Times reporter was photographed by men with mobile phone cameras at least three times last week but was never directly challenged.

“It’s like a web — they let you spin away and like that they believe they get more information,” said the French intelligence source.

The presence of minders made asking questions around Ain Aouda almost impossible, but at a restaurant adjoining a newly built mosque nearby, elderly men supping mint tea while they watched the African Nations Cup were clearly angry about the project.

“We’ve seen nothing but Americans for five months,” complained one wizened figure before being told by his friends to be quiet.

There are no public access roads leading to the site, which can be seen at the moment only from a bend above the Korifla Gorge. The secluded forest setting is similar to that of Temara, 10 miles to the north.

Moroccan officials refused to make any comment about Ain Aouda. A spokeswoman for the American embassy said she had no information about the new building.

Temara itself already has a fearsome reputation among former inmates. Binyam Mohammed, an Ethiopian-born Briton later sent to Guantanamo Bay, told Amnesty International that interrogators there cut his chest and penis when he refused to answer questions.

Mohammed said he was held at Temara for 18 months before being flown to another “black prison” in Afghanistan in January 2004, and then on to Guantanamo Bay.

It is not clear how many suspects are being questioned in Morocco. The French intelligence source said the four brought to the country in December were all believed to be “high profile” but gave no further details.