A checkpoint shooting in eastern Afghanistan has taken the US military’s death toll in the war past 2,000.
A US soldier and contractor were killed while three Afghan soldiers died and several were injured.
The new deaths occurred on Saturday in Wardak province.
The international mission, ISAF, initially said the soldier was believed to have been killed by a member of the Afghan security services, but it later said the circumstances were unclear.
What is known is that a firefight took place, after what ISAF described as a short conversation between coalition and Afghan soldiers.
ISAF says “insurgent fire” may have been involved in the incident, which is now under investigation by a joint Afghan and coalition team.
The American death toll goes back to the US-led invasion of Afghanistan in 2001.
Sunday’s incident took place at a checkpoint near an Afghan National Army base in the district of Sayedabad, according to Afghan officials.
Shahidullah Shahid, a provincial government spokesman, earlier told the Associated Press news agency that an Afghan soldier had turned his gun on Americans and started shooting.
“Initial reports indicate that a misunderstanding happened between Afghan army soldiers and American soldiers,” he said.
But ISAF later said an American soldier and an American contractor, along with three Afghan soldiers, were killed in an exchange of fire in confusing circumstances that may have involved insurgent activity.
ISAF’s deputy commander, Lt Gen Adrian Bradshaw, said: “The circumstances were somewhat confused and we are establishing the full facts to the extent that it is possible.”
Military officials from both sides have launched a joint investigation.
The figure of 2,000 deaths was given by US officials on Sunday. During the war in Iraq, 4,409 American soldiers were killed.
As of 27 September, the Pentagon’s official military death toll for Afghanistan had stood at 1,996.
The count includes both soldiers killed in action and soldiers who died of their injuries in hospital. The figure also covers 339 non-combat deaths.
A report by the Brookings Institution estimates that 40.2% of US deaths were caused by improvised explosive devices and 30.3% by gun attacks.
Officially, at least 17,644 US soldiers have been wounded in action in Afghanistan.
The independent organization iCasualties estimates a higher US death toll, recording 2,125 to date.
This same source reports 1,066 deaths of non-US members of the coalition in Afghanistan.
It is more difficult to establish the Afghan toll in the war but most estimates calculate a minimum of 20,000 civilian deaths, AP notes.
Some 10,000 members of the Afghan security forces have been killed. No reliable figures exist for deaths among the Taliban and other insurgents.
NATO combat troops are set to withdraw by the end of 2014, but a central plank of the strategy is that foreign soldiers will serve alongside and train Afghans for many years to come.
Correspondents say that may not be realistic given the ever increasing number of Afghans who turn their weapons on their foreign allies.
The American soldier who shot dead 16 civilians, including women and children, in Afghanistan on Sunday has been flown to Kuwait, US officials say.
Afghan MPs have demanded that the man be tried in Afghanistan, but the scenario is very unlikely.
Meanwhile an Afghan man who crashed a lorry at an airfield as the US defense secretary’s plane was arriving has died of his injuries, officials say.
US Defense Secretary Leon Panetta was not at risk at any time, US officials said.
Lt. Gen. Curtis Scaparrotti said the man had apparently tried to ram the stolen vehicle into a group of US Marines at Camp Bastion in Helmand.
Leon Panetta was in Helmand to address US troops, as fears mount that they could be the target of a backlash against foreign forces.
He also met Afghan President Hamid Karzai in an effort to rebuild relations rocked by incidents such as the massacre in Kandahar and the burnings of Korans at a US military base last month.
The attack in Kandahar province has caused outrage across Afghanistan and protests in several areas. On Thursday about 2,000 people demonstrated in the southern Afghan province of Zabul, the second major protest outside Kandahar this week.
NATO has insisted that the detained man carried out the killings on his own.
But the head of an Afghan parliamentary delegation said he has heard evidence from local villagers which suggests as many as 20 US soldiers were involved.
About 2,000 people demonstrated in the southern Afghan province of Zabul after the US soldier shot dead 16 civilians in Kandahar
Sayed Ishaq Gillani, a leading Afghan MP, also claims that helicopters were heard overhead, and that they were seen dropping chaff – a measure designed to protect aircraft from ground attack.
Sayed Ishaq Gillani said local people believe the killings were carried out in revenge for an attack a week earlier in which several US troops were hurt.
The victims were shot in their homes in the remote Panjwai district of Kandahar, which is also the spiritual homeland of the Taliban.
According to the NATO version of events, the staff sergeant, who has not been named or charged, allegedly left his base in southern Afghanistan before dawn on Sunday, entered several houses in the area and shot men, women and children at close range.
The soldier was held by the US military in Kandahar until Wednesday evening, when he was flown out of the country “based on legal recommendation”, a Pentagon spokesman, Captain John Kirby said.
“We do not have appropriate detention facilities in Afghanistan,” Captain John Kirby said.
A NATO official later confirmed that the suspect had been flown to Kuwait.
Members of the Afghan parliament had demanded that he should be put on trial in their country.
But this was never going to happen. The US has always insisted that charges of wrongdoing by its soldiers be dealt with within the American military legal system.
US officials say the soldier handed himself in. Leon Panetta has said that if found guilty, he could face the death penalty.
Officials said the soldier had completed several tours in Iraq but was on his first tour of duty in Afghanistan.
NATO and the US administration have insisted that there will be no change of strategy in Afghanistan. The issue has been at the top of the agenda as the UK’s Prime Minister David Cameron makes a state visit to Washington.
NATO’s International Security Assistance Force (ISAF) plans to withdraw all of its combat forces by the end of 2014. American troops are also following that timetable.