A roadside bomb killed three US soldiers in south Afghanistan on Tuesday, officials said, the latest fatalities for the NATO-led coalition which is winding down after 12 years of fighting.

The soldiers died in the volatile province of Kandahar, where five US troops were killed by a similar improvised explosive device targeting a vehicle ten days ago.

Kandahar is a hotbed of insurgent activity, with large areas under the control of Taliban militants who were ousted from power in Kabul in 2001 after the 9/11 attacks on the United States.

"We can confirm that three soldiers were killed in a roadside bomb in southern Afghanistan today," Major Bryan Purtell, a spokesman for the NATO International Security Assistance Force, said.

Purtell said an earlier death toll of four had been revised down and that recovery work was still ongoing at the blast site.

Javid Faisal, spokesman for the provincial government of Kandahar, told AFP that the victims were American.

"At around 2:30 pm, a NATO armoured vehicle hit a roadside mine in Zhari district," he said.

Before Tuesday's attack, 46 US soldiers and 13 troops from other coalition nations have died in Afghanistan this year, according to the independent "icasualties" website.

Three Georgian soldiers were killed in neighbouring Helmand province on Monday when a suicide bomber detonated a vehicle laden with explosives.

Also on Tuesday, Taliban insurgents announced they had released the final four Turkish engineers held hostage since their helicopter crash-landed in the eastern province of Logar last month.

"The remaining four Turkish nationals were released today as a sign of goodwill and as an Islamic and humanitarian gesture of respect to the Muslim Turkish nation," the insurgents said in an emailed statement.

"They were delivered to relevant Turkish authorities."

Four other Turkish engineers on board the helicopter were freed on Sunday.

The Islamist militants made no reference to a Russian, a Kyrgyz national and an Afghan also taken hostage after the crash-landing on April 21 in Logar, a Taliban stronghold south of the capital Kabul.

The seizure was the largest abduction of foreigners in almost six years and highlighted Afghanistan's continuing insecurity as NATO combat troops prepare to pull out next year.

Turkey, one of only two Muslim-majority members of NATO, has around 1,800 soldiers serving with the US-led military coalition, but they do not take part in combat operations.

The Taliban launched their annual "spring offensive" on April 27, opening a crucial period for the country as its security forces take the lead in offensives against the insurgents.

All NATO combat missions will finish by the end of next year and the 100,000 foreign troops deployed across Afghanistan have already begun to withdraw from the battlefield.