Irbil, Friday December 24, 2004 - Insurgents have been able to "operate at will" in Mosul, where 22 people died in a bomb attack this week, because the US forces and the Iraqi authorities have failed to tackle them, an intelligence assessment by senior US officials in northern Iraq concludes.

The report, seen by the Guardian yesterday, was drafted before this week's suicide attack on the mess tent at Camp Merez.

It was made after the uprising last month, when most of Mosul's police force either deserted or defected and parts of the city fell, albeit briefly, to the insurgents.

It does not specifically mention threats to US bases, but it catalogues a series of errors and missed opportunities in intelligence gathering, recruitment to the Iraqi security forces, and operational issues.

Its assessments and recommendations reflect many of the concerns expressed in briefings of the Guardian recently by General Carter Ham, the US commanding officer in Mosul, and senior Iraqi officials in the city.

Tuesday's explosion was the worst single attack on the US forces since the invasion in March last year, and has thrown Pentagon officials and the defence secretary, Donald Rumsfeld, on to the defensive, on account of their apparent failure to protect the US troops in Iraq adequately.

General Richard Myers, the chairman of the joint chiefs of staff, said on Wednesday that a suicide bomber had probably strapped an explosive device to his body and entered the metal and canvas dining hall where the explosion occurred.

The attacker may have had knowledge of the base's layout and the meal-time schedules.

The military authorities said yesterday that they had begun re-examining the security measures at bases throughout Iraq.

"We always have force protection keeping their eyes out," Lieutenant Colonel Paul Hastings, the spokesman for the US forces in Mosul, told reporters.

"For somebody that wants to take his life and kill himself - it's very difficult to stop those people."

Ansar al-Sunna, a militant Sunni Islamist group, claimed responsibility for the attack, saying it had infiltrated a suicide bomber into the base.

The ability of insurgent groups to penetrate the Iraqi security forces in many areas of the Sunni triangle has been a major source of concern for US forces.

After the November violence in Mosul a former chief of police in the city was arrested by Kurdish officials on suspicion of handing over police stations to insurgents.

Since then there has been a spate of murders in and around the city of members of the Iraqi national guard, their killers apparently acting on inside information.

But Iraqis also enter US bases as employees of civilian contractors providing services such as laundry, catering, and refuse collection. Jeremy Redmon, a reporter from the Richmond Times-Dispatch embedded with the troops at the Mosul base, told CNN yesterday that Iraqi workers had to show identification to get in but could then move with relative freedom.

The intelligence assessment suggests there was a lack of rigour in the vetting procedures for posts in the Iraqi security forces.

It says: "The US military and Iraqi government should have been aware of the history of persons they appointed to positions."

The lack of effective action by the interim Iraqi government "left the door open for the terrorist groups to work freely in secure areas", it says, adding that better vetting is essential."Each appointment must be seriously reviewed."

The report outlines the scale of the problem facing the US and Iraqi authorities as they try to maintain order in the city before the general election. It says Mosul is a candidate to become an alternative to Falluja.

"They [the militants] have enough supporters there who can hide them, and also the Syrian border is close."

It says that many residents are buying weapons in prepa ration for the next outbreak of violence and that it is vital for the interim government to build trust with - and spend money on - the people of Mosul and for the US forces to treat them with respect.

· At least three US marines were killed in renewed fighting in Falluja yesterday as refugees from the original US assault on the city began returning to their homes.

The US forces bombed and shelled suspected insurgent positions.

Some of the estimated 250,000 people who fled the city queued at Iraqi police checkpoints to show their documents before returning to the remains of bombed and looted houses, some of which still contain the bodies of those killed in recent weeks.