The attacks drew unusually strong criticism Sunday from the U.S. military's top officer, Army Gen. 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 supposed Afghan allies or those who have infiltrated their ranks.
At least 12 such attacks occurred 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. military 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 United States will have 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."
Adding to the tension was the continuation of protests across the Muslim against an Internet video mocking the Prophet Mohammad.
In unusually blunt remarks to the Pentagon's own news service, General Dempsey said the Afghan government needs to take the problem as seriously as do U.S. commanders and officials.
"We're all seized with [the] problem," the general said 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 Friday night, when 15 insurgents disguised in U.S. army uniforms killed two Marines and wounded nine other people at Camp Bastion, a major U.S. base in the south, military officials said.
It was one of the worst attacks on a NATO-operated base all year. Six Harrier jets were destroyed and two were significantly damaged in the raid on the camp's air field, the NATO-led coalition said Sunday.
Operating in three teams, the attackers succeeded in breaching the perimeter of the heavily fortified base. Britain's Prince Harry was at Camp Bastion at the time of the attack, but he was unharmed.
Three refuelling stations also were destroyed and six aircraft hangars were damaged by the attack. All but one of the attackers were killed. The remaining fighter taken into custody by coalition forces.
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. "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, Mr. 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 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, Chief 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," the deputy chief said.
He said all four of the dead were American, as did a U.S. official speaking on condition of anonymity.
General Dempsey said that the Friday assault on Camp Bastion, a British air base in Helmand province, should not be called an insider attack even though the attackers were wearing U.S. Army uniforms. An initial review found the attackers had no inside assistance, he said. It was not clear how the attackers got the U.S. uniforms.
The latest deaths make at least 247 American troops killed in Afghanistan so far this year.
Nearly 2,000 American troops have been killed in the conflict since the 2001 invasion.
Taliban spokesman Qari Yousef Ahmadi said the police who attacked were not affiliated with the Taliban insurgency. "But they are Afghans and they know that Americans are our enemy," Ahmadi said in an emailed statement. He said the police who fled have joined up with the insurgency.
The coalition said in a statement that they were investigating what happened.
Elsewhere in Afghanistan, a NATO airstrike killed eight women and girls gathering firewood in the remote Laghman province.
The incident 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 International Security Assistance Force, as the U.S.-led coalition is known, acknowledged that civilians had been killed and expressed its regret over the airstrike.
It insisted known insurgents had been the target.
Capt. Dan Einert, a spokesman for NATO forces, said that the strike killed as many as 45 insurgents but also may have killed five to eight Afghan civilians.
Mr. Karzai "strongly condemns the airstrike by NATO forces which resulted in the deaths of eight women," a statement from his office said.