Afghanistan's security service denied claims that a suicide bomber who killed two American troops and four Afghans in a weekend attack was a member of the country's intelligence agency.

The remarks Tuesday contradict statements by U.S. and local officials who had described the assault as an insider attack and the first of its kind by a member of Afghanistan's elite intelligence service.

The growing number of insider attacks has raised doubts whether Afghan forces will be able to effectively control security in their country after foreign troops depart in 2014.

In the attack Saturday, the bomber blew himself up as a group of Americans and Afghan officials were arriving to deliver new office furniture to the intelligence headquarters in Kandahar's Maruf district. Two American troops and four Afghan intelligence agents died in the blast.

The NATO military coalition and local officials said the bomber was a member of Afghanistan's National Directorate of Security, or NDS.

"The attacker was wearing an NDS uniform, and was an NDS employee," coalition spokesman Maj. Adam Wojack said in an email.

But Shafiqullah Tahiri, spokesman for the intelligence agency, said Tuesday that the attacker had worn an Afghan uniform to gain access to the building in Kandahar.

"The suicide bomber was never on the staff of the NDS," he said. "Unfortunately, the local media reported this. It created concern among the people of Afghanistan."

Afghan authorities are increasingly sensitive to the spate of insider attacks on foreign troops and members of the Afghan army and police. The government has sought to play down the significance of these attacks, which have raised concerns about the level of Taliban infiltration in Afghanistan's security forces.

So far this year, more than 50 U.S. and NATO troops have died at the hands of their would-be Afghan partners. But Saturday's bombing was the first reported insider attack by an Afghan intelligence agent, according to logs kept by The Associated Press and the international military coalition.

This would raise the scope of insider attacks to a new level since members of the security agency are much more closely vetted than the Afghan army or police.

Although the coalition maintains that Taliban insurgents are responsible for only a minority of the insider attacks, it is already clear that the tactic — virtually unknown in previous guerrilla wars — has seriously undermined the confidence of Western nations in the Afghan government's ability to control the security situation after 2014.

It has also increased calls in allied nations for a possible speeded up withdrawal of forces. Earlier this week, the British government said thousands of its 9,500 troops will leave Afghanistan next year, a major reduction in the U.K. forces in the country.

During a visit Tuesday, the British foreign service officer for Afghanistan, Sayeeda Warsi, stressed that her country is not abandoning its Afghan ally but would remain one of Kabul's "biggest supporters" after the pullout.

"Our relationship with Afghanistan is not limited to combat troops," Warsi said. "Therefore, when our combat troops leave at the end of 2014, that relationship will continue on a whole series of levels which include support in terms of training for security forces."

In other guerrilla wars, insurgents have been known to plant agents among security forces mainly to glean intelligence on their plans and, to a lesser extent, obtain weapons, ammunition, medical and other supplies.

Historians say insider attacks were virtually unheard of during the U.S. involvement in Vietnam, the French war in Algeria, or the Soviet occupation of Afghanistan in the 1980s.

Meanwhile, NATO acknowledged that its forces may have accidentally killed three Afghan children on Sunday in Helmand province while targeting suspected insurgents laying roadside bombs.

The coalition expressed regret for the deaths and said its representatives would meet with family members to express condolences. Afghan media has reported that besides the three children, several insurgents were also killed in a NATO airstrike. But a NATO statement denied that close air support was involved in the incident.


Associated Press writers Heidi Vogt and Slobodan Lekic in Kabul contributed to this report.