Last Updated 3:40 p.m. ET
(AP) KABUL, Afghanistan- Afghan police killed four American soldiers coming to their aid after a checkpoint attack Sunday, the third "insider" 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 in a remote part of the country strained the military partnership between Kabul and NATO as the U.S. begins to withdraw thousands of troops sent three years ago to route the Taliban from southern strongholds.
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.
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 U.S. 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 U.S. will remain with about 68,000 troops at the end of September.
NATO and U.S. 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."
The airstrike that killed the eight women and girls, meanwhile, drew an apology from the U.S.-led coalition, condemnation from Afghan President Hamid Karzai and cries of "Death to America!" from villagers who retrieved the bodies.
The insider attacks began Friday night, when 15 insurgents disguised in U.S. army uniforms killed two Marines, wounded nine other people and destroyed six Harrier fighter jets at a major U.S. 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 district 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.
"It was my understanding that it was a checkpoint," said Jamie Graybeal, a spokesman for international military in Afghanistan.
One police officer was killed in the clash with NATO troops, he said. Other officers at the site fled; it was unclear if they were involved in the attack or not.
Two international troops were wounded and were receiving treatment, Graybeal said. He did not say how serious the injuries were.
Afghan officials said the checkpoint in Zabul's Mizan district came under attack first 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.
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.
It was not clear if some of the Afghan police turned on the Americans in the middle of the battle, or were somehow forced into attacking the American troops by the insurgents, 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," Gilani said.