The Ministry of Defence (MoD) said two British soldiers from 3rd Battalion The Yorkshire Regiment were shot by a man wearing the local Afghan police uniform at a checkpoint in the south of Nahr-e Saraj district in Helmand province yesterday.
The deaths follow that of a soldier from 1st Battalion Grenadier Guards who died on Friday after his vehicle hit a roadside bomb. Next of kin have been told.
They were the latest in a string of insider attacks by Afghan forces against their international counterparts. The killings threaten the military partnership between Kabul and Nato, a working relationship that is key to the handover of security responsibilities to Afghan forces as international troops draw down.
Meanwhile, Afghan government officials said a Nato airstrike killed eight women and girls who were out gathering firewood before dawn on Sunday in a remote region in the east of Afghanistan.
Jamie Graybeal, a spokesman for the international military in Afghanistan, said details of the US attack were slow to come out because it took place in a remote region. He said the attacker was still at large but it was unclear if there were multiple assailants.
"The attack took place in the vicinity of an outpost in southern Afghanistan. It is my understanding that it was a checkpoint," Mr Graybeal said.
International forces often work with Afghan police to man checkpoints as part of the effort to train and mentor the Afghan forces so that they can eventually operate on their own. The goal is to turn over all security responsibility for the country to the Afghans by the end of 2014, though numbers of Nato forces have already been reduced in some areas.
Afghan officials said the checkpoint, in Zabul province's Mizan district, first came under attack from insurgents sometime around midnight. American forces came to help the Afghan police respond to the attack, said Ghulam Gilani, the deputy police chief of the province.
It was not clear if some of the Afghan police turned on their American helpers in the middle of the battle with the insurgents, or afterward, or were somehow forced into attacking the American troops by the insurgents, Mr Gilani said.
"The checkpoint was attacked last night. Then the police started fighting with the Americans. Whether they attacked the Americans willingly we don't know," Mr Gilani said.
He said all four of the dead were American. The coalition said in a statement that they are investigating what happened.
There were also international troops wounded, Mr Graybeal said. He said that early reports showed two were hurt and that they were receiving treatment. He did not say how serious the injuries were.
So far this year, 51 international service members have died at the hands of Afghan soldiers or policemen or insurgents wearing their uniforms. At least 12 such attacks came in August alone, leaving 15 dead.
Meanwhile, the full scale of the Taliban attack on Camp Bastion shortly after 10pm on Friday became clear today as it emerged that 15 insurgents wearing US Army uniforms destroyed six Harrier jets, three refuelling stations and damaged a number of aircraft hangars.
As well as the two US marines who were killed, nine coalition staff were wounded in the attack, Isaf, the International Security Assistance Force coalition in Afghanistan, said.
Coalition forces killed 14 insurgents and wounded one other, who was then taken into custody.
The attack happened when Taliban fighters, organised into three groups and armed with automatic rifles, rocket-propelled grenade launchers and suicide vests, breached the camp's perimeter.
The offensive took place near an airfield on the north-east side of the base, which houses American forces in Camp Leatherneck.
An Isaf spokesman said: "The insurgents appeared to be well equipped, trained and rehearsed.
"Dressed in US Army uniforms and armed with automatic rifles, rocket propelled grenade launchers and suicide vests, the insurgents attacked coalition fixed and rotary wing aircraft parked on the flight line, aircraft hangars and other buildings."
The official said the six Harrier jets destroyed were US marine aircraft and that two others were significantly damaged.
Six aircraft hangars were also damaged.
The nine personnel who were wounded included eight military and one civilian contractor, Isaf said. None of the injuries is thought to be life-threatening.
The Taliban has claimed responsibility for the attack on Camp Bastion, saying saying it was carried out because Prince Harry was on the base, and also as revenge for an anti-Islamic film.
The Prince was unharmed and defence experts said yesterday that he should not be withdrawn from his military role in Afghanistan, despite the attack.
Harry, an Army captain, is based at Camp Bastion for his second tour of duty to the country, which is due to last four months.
He was about two kilometres away with other crew members of the Apache attack helicopters, of which he is a co-pilot gunner, when the attack took place, sources said.
Tory MP Colonel Bob Stewart, a former commander of British troops in Bosnia, said he did not think the Prince should be pulled out of Afghanistan because of the attack by the Taliban.
"To hell with them," he said. "Harry wants to go there and our soldiers want him there. He should stay."
But Col Stewart stressed the security considerations regarding the deployment of the Prince were flexible.
"These things aren't set in concrete. If circumstances really change then we'll make different judgments."
He added: "Capturing, killing or hurting Prince Harry would be a huge propaganda coup for the Taliban."
Qari Yousef Ahmadi, a spokesman for the Taliban, told The Associated Press: "We attacked that base because Prince Harry was also on it and so they can know our anger."
He added: "Thousands more suicide attackers are ready to give up their lives for the sake of the Prophet."
Major Charles Heyman, a former infantry officer and editor of The Armed Forces of the United Kingdom publication, warned against the dangers of "playing into the hands of the Taliban".
He said: "On balance it is a difficult equation but I think he should be kept there.
"If we take him away the Taliban will crow that they have just scored a major victory.
"The second point is it would affect the morale of the troops on the ground if Prince Harry was taken out just because there was a threat."
Maj Heyman added that it was not the first time Camp Bastion had been targeted.
"The Taliban have been doing these things for five years now," he said.
"Bastion is a huge complex. It is really a military city. This is one of those pinprick attacks that went right as they killed two US Marines. But in most of them no-one gets hurt."
A Ministry of Defence spokesman said: "The threat to all our service personnel is continually assessed and all measures taken to mitigate it.
"As we stated last week, the deployment of Captain Wales has been long planned and the threat to him and others around him thoroughly assessed.
"We stated that any risk posed by his deployment, based on the capability, opportunity and intent of the insurgency, is continually reviewed."
An anti-Islamic film sparked a series of protests and violence across the Muslim world this week. An MoD source said: "After saying this attack was mounted in reaction to the video on Islam, it is entirely predictable that the Taliban have changed their tune to say it was aimed at Captain Wales.
"The insurgency who mounted this attack - most of who were killed by Isaf - were nowhere near Captain Wales, who with other UK and Isaf personnel was under lockdown."
Harry, who celebrated his 28th birthday today, arrived in Afghanistan on September 7.
He has been undergoing training to fly operations in Apache attack helicopters and is expected to start flying missions this week as a co-pilot gunner.
Camp Bastion is a huge base in the middle of the desert and is shared with US, Estonian, Danish and Afghan troops.
It is the logistics hub for operations in Helmand, with supply convoys and armoured patrols regularly leaving its heavily-defended gates, to support the military forward operating bases, patrol bases and checkpoints spread across the province.
Villagers from Laghman province's Alingar district took the bodies of the dead women to the governor's office in the provincial capital, said Sarhadi Zewak, a spokesman for the provincial government.
"They were shouting 'Death to America!' They were condemning the attack," Zewak said.
Seven injured females were taken to area hospitals for treatment, some of them as young as 10, according to provincial health director Latif Qayumi.
Nato forces at first said that about 45 insurgents and no civilians were killed in the attack but spokesman Jamie Graybeal stressed later that they took the charge of civilian deaths seriously and were investigating the allegations. He said, however, that initial reports showed only insurgents were killed in the airstrike.
Airstrikes have been a particularly sensitive issue between the Afghan people - who say civilians often end up killed along with or instead of insurgents - and Nato forces, who maintain that they are a key tactic for going after insurgent leaders.