Afghans fatally shoot 2 US troops at joint base Updated at 7:54 a.m. ET: KABUL, Afghanistan -- Two American soldiers were killed Thursday in a shooting by an Afghan soldier and a literacy teacher at a joint base in southern Afghanistan, officials said, the latest in a series of deaths as anti-Americanism rises following the burning of Qurans by U.S. soldiers. Both were killed on the same day that the top NATO commander allowed a small number of foreign advisers to return to work at Afghan ministries after more than a week of being locked down in secure locations because of the killing of two other Americans. Thursday's killings raised to six the number of Americans killed in less than two weeks amid heightened tensions over the Feb. 20 burning of Qurans and other Islamic texts that had been dumped in a garbage pit at Bagram Air Field near Kabul. More than 30 Afghans also were killed in six days of violent riots that broke out after the incident. President Barack Obama and other U.S. officials apologized and said the burning was an accident, but that has failed to quell the anger. "Two individuals, one believed to be an Afghan National Army service member and the other in civilian clothing, turned their weapons indiscriminately against International Security Assistance Force and Afghan National Security Force service members in southern Afghanistan today," the statement said. A senior defense official confirmed to NBC News that both of the NATO service members were American. The Associated Press quoted a U.S. official as saying three attackers were believed to be involved, two of whom were subsequently killed. He said the third may be in custody. The official spoke on condition of anonymity because he was not authorized to speak on the record. A district chief in southern Kandahar's Zhari district said the shootings took place on a NATO base when an Afghan civilian who taught a literacy course for Afghan soldiers and lived on the base started shooting at NATO troops. Niaz Mohammad Sarhadi said the shootings occurred at 3 a.m. and that NATO troops returned fire and killed the man and an Afghan soldier. Mohammad Mohssan, an Afghan Army spokesman in Kandahar city, confirmed the incident occurred at a base in Zhari and involved two Afghans, one of whom was a soldier, who opened fire on coalition troops from a sentry tower. He said both were killed. The shootings on Thursday were the latest in a series of attacks by Afghan security forces - or militants disguised in their uniforms - against Americans and other members of the international alliance. Last month the Pentagon released data showing that 75 percent of the more than 45 insider attacks since 2007 occurred in the last two years. They are likely to raise further questions about the training of Afghan security forces by coalition troops as foreign forces prepare to withdraw by 2014. Hundreds of advisers were pulled out of ministries and other government locations after an Afghan gunman shot and killed two U.S. military advisers on Feb. 25 inside their office at the Interior Ministry. The Taliban claimed responsibility for the ministry shootings, saying they were conducted in retaliation for last week's Quran burnings, but no one has been arrested in the case. An Afghan soldier also killed two U.S. troops in eastern Afghanistan on Feb. 23 during a protest over the Quran burnings. U.S. military spokesman Lt. Col. Jimmie Cummings said Thursday that Marine Gen. John Allen, the top commander in Afghanistan, approved the return of selected personnel. He could not elaborate which ministries were involved, but an Afghan official said some had returned to a department setting up a government-run security force that will guard international development projects. A NATO official said less than a dozen advisers had returned. Both officials spoke on condition of anonymity because of the sensitivity of the issue. Foreign advisers are key to helping improve governance and prepare Afghan security forces to take on more responsibility. The U.S. is already reducing its own troop presence by 30,000 at the end of the summer. Many of the remaining soldiers will switch from fighting to training and mentoring Afghan forces. NBC News' Courtney Kube, The Associated Press and msnbc.com staff contributed to this report.