Six American soldiers training Afghan police in a Taliban flashpoint were shot dead by one of their students in the deadliest such incident in at least two years, officials said Tuesday.

The shooting, which follows a string of similar attacks on Nato troops, underscores the challenges faced by the US-led mission as it aims to build the national army and police to take responsibility for security by 2014.

Nato's International Security Assistance Force (ISAF) said the gunman, identified as a local resident who had been in training with Western troops for only a few days, was also killed, and an investigation was underway.

Wearing Afghan border police uniform, the man turned his weapon against ISAF troops during the training mission on Monday, killing the six soldiers in Taliban-infested eastern Afghanistan, Nato announced.

A US defence official identified the six troops as Americans but declined to give further details. Ahmad Zia Abdulzai, spokesman for the governor of Nangarhar province where the incident took place, named the gunman as Azat Gul, a policeman being schooled at the training centre in the Pachir Agam area of Khogyani district.

"After a few days of training, yesterday he opened fire and killed six American soldiers. The incident is under serious investigation," Abdulzai told AFP.

Americans make up most of the foreign troops based in eastern Afghanistan, one of the worst flashpoints in the nine-year Taliban insurgency.

The deaths bring the number of coalition troop fatalities in 2010 to 668, according to an AFP tally based on that tracked by the independent icasualties.Org website.

It is the highest annual toll since the US-led invasion in late 2001. Last year 521 Nato soldiers died. A Taliban spokesman praised the attack in emailed comments carried by the SITE Monitoring service.

Naming the attacker as Izzatullah, the spokesman said he had shot dead six US soldiers and wounded several others before being killed in return fire.

The Taliban have made unsubstantiated claims of attacks on Nato forces. The United States is bankrolling a massive programme — $9.2 billion dollars in fiscal 2010 — to build Afghanistan's army and police so they can take over responsibility for security by 2014, as pledged by Nato in Lisbon a week ago.

But the programme has been troubled by a series of shootings, either by insurgents dressed in Afghan security uniforms or rogue officers. An ISAF spokesman said Monday's incident was the deadliest of its type since their database was started more than two years ago.

This month Nato said it was investigating whether an Afghan soldier killed two coalition troops on a military base in the volatile town of Sangin in southern Helmand province.

In July, an Afghan soldier killed two American contractors inside a military base in north Afghanistan.

A week later another Afghan solider killed three British Gurkha soldiers. In 2009, five British soldiers were killed by an Afghan policeman. The police are seen as central to the goal of handing Afghans the lead in the fight against the Taliban, who were ousted by the US-led invasion in late 2001 but wage an increasingly deadly guerrilla war.

There are about 80,000 police officers and US and Nato forces hope to bring that number up to 134,000 by October 2011, alongside the 170,000 personnel planned for the army by the same date.

Leading think tank the International Crisis Group has issued a damning review of the war, saying Afghan security forces "have proven a poor match for the Taliban" and were "corrupt, brutal and predatory".