Afghan police killed four American soldiers coming to their aid after a checkpoint attack - the third assault by government forces or insurgents disguised in military uniforms in as many days.
The escalating violence - including a Nato airstrike that killed eight Afghan women and girls gathering firewood - is straining the military partnership between Kabul and Nato as the US begins to withdraw thousands of troops sent three years ago to get rid of the Taliban from southern strongholds.
The attacks drew unusually strong criticism from the US military's top officer, Army General Martin Dempsey, who called the problem of rogue Afghan soldiers and police turning their guns on allied troops "a very serious threat" to the war effort.
This year, 51 international service members have died at the hands of their Afghan allies or those who have infiltrated their ranks. At least 12 such attacks came in August alone, leaving 15 dead.
The surge in insider attacks is a sign of how security has deteriorated as Nato prepares its military exit from Afghanistan by the end of 2014. The US is days away from completing the first stage of its own drawdown, withdrawing 33,000 troops that were part of a military surge three years ago. The US will remain with about 68,000 troops at the end of September.
Nato and US forces are working with the Afghan government to tighten vetting procedures and increase security between the forces, but nothing has so far been able to stem the attacks on troops, which Nato frequently asserts are standing "shoulder by shoulder."
In unusually blunt remarks to the Pentagon's own news service, the American Forces Press Service, Gen Dempsey said the Afghan government needs to take the problem as seriously as do US commanders and officials. "We're all seized with (the) problem," said Dempsey, after discussing the issue at a meeting in Romania with Nato officials. "You can't whitewash it. We can't convince ourselves that we just have to work harder to get through it. Something has to change. We have to get on top of this. It is a very serious threat to the campaign."
A weekend of deadly attacks began on Friday night, when 15 insurgents disguised in US army uniforms killed two Marines, wounded nine other people and destroyed six Harrier fighter jets at a major US base in the south, military officials said. On Saturday, a gunman in the uniform of a government-backed militia force shot dead two British soldiers in Helmand province in the southwest.
On Sunday, an Afghan police officer turned his gun on Nato troops at a remote checkpoint in the southern province of Zabul, killing four American service members, according to Afghan and international officials. One police officer was killed in the clash with Nato troops. Two international troops were wounded and were receiving treatment.
The attacks come amid an international uproar about an internet video mocking the Prophet Muhammad that many fear could further aggravate Afghan-US relations. The video has sparked protests throughout the Muslim world and the Afghan government blocked the YouTube site that hosts the video and its parent company, Google, over the weekend in a move to prevent violent protests. The Taliban claimed that Friday's assault on Camp Bastion was revenge for the video insulting the prophet.