Two American soldiers were killed by Afghan colleagues on Thursday, the latest in a series of such attacks after the burning of Korans at a US base sparked widespread violent protests.

They were killed at a military outpost in the restive southern province of Kandahar, the spiritual birthplace of the Taliban, and two of the three gunmen responsible for the killings were Afghan government troops, the Pentagon said.

The attack raised the death toll to six Americans killed by Afghan associates in the week since angry protests broke out over the Koran burning incident at a US military base at Bagram near the capital Kabul.

"We believe that two individuals from ANSF (Afghan security forces) were involved in the attack and a civilian Afghan as well," said Pentagon press secretary George Little.

The civilian was a literacy teacher working in the outpost who grabbed a weapon from a soldier and opened fire, Zhary district chief Niaz Mohammad Sarhadi told AFP.

Little said that "two of the attackers were subsequently killed by (NATO) forces" but the fate of the civilian assailant was still unknown.

NATO withdrew all its advisors from Afghan government ministries on Saturday after two American officers were shot and killed inside the interior ministry, apparently by an Afghan colleague.

Two days earlier, two American troops were killed by an Afghan soldier who turned his weapon on them as demonstrators approached their base in the east of the country.

Of the 60 deaths of NATO troops this year, almost one in five has been at the hands of Afghan colleagues -- including up to six Americans, four French and an Albanian.

Before the Koran burning, the Pentagon reported 42 "blue-on-green" attacks -- involving Afghan soldiers -- since 2007 that left about 70 ISAF coalition troops dead and approximately 110 wounded.

An Afghan source working with ISAF told AFP "an investigation was ongoing" into whether the latest killings were connected to the burning of the Koran.

Popular outrage erupted after Afghans learned that copies of Islam's holy book were thrown into an incinerator pit at the US-run Bagram airbase, leading US President Barack Obama to apologise for what he described as an error.

Some 40 people have been killed in six days of violent demonstrations as protesters targeted Western bases, plunging relations between US-led Western forces and their Afghan allies to an all time low.

The UN pulled international staff out of their base in the northern province of Kunduz after it came under attack Saturday by demonstrators, while on Sunday seven US soldiers were wounded in a grenade attack in the same province.

The UN said Thursday the perpetrators of the Koran burning should be punished, but insisted ties between the international community and the Afghan people would emerge stronger.

"It hasn't affected our determination to work with the people of Afghanistan and the authorities," the special representative for the UN in Afghanistan Jan Kubis told a news conference.

NATO has 130,000 troops fighting the Taliban, which has led an insurgency against the Western-backed Kabul government since being toppled from power in 2001.

Obama said Wednesday he believed the United States could stick to its Afghan drawdown timetable despite the unrest.

"I feel confident that we can stay on a path that by the end of 2014, our troops will be out and will not be in a combat role and Afghans will have capacity, just as Iraqis, to secure their own country," Obama said.

However, the attack on the US officers within the heavily-fortified interior ministry in Kabul "has hit at the heart of cooperation between the international community and the Afghans", said Kate Clark of the Afghanistan Analysts Network.

"For the US and NATO strategy to go ahead they need to turn out trained Afghan forces, and if their advisors and trainers are not safe it raises a lot of questions about whether they can carry out that strategy," she told AFP.