Kabul, Afghanistan (CNN) -- Eight U.S. service members and an American civilian contractor were killed Wednesday in a shooting at an Afghan air force compound in Kabul, officials said.
The NATO-led International Security Assistance Force in Afghanistan said an Afghan military pilot opened fire on international troops, sparking a "gunfight." The Taliban, however, claimed responsibility for the attack and said it had been working with the shooter for some time -- an assertion that NATO denied.
Also denying the Taliban claim was the brother of the pilot.
"My brother had no connections with the Taliban, and I deny any claims of his connection by the Taliban," Dr. Mohammad Hosain Sahebi told a local Afghan TV station in a telephone interview.
He said his brother, Ahmad Gull, 48, was in the Afghan Air Force for several years and was injured many times in plane crashes. The Afghan military, however, listed the pilot as being 50 years old.
"My brother had mental sickness as the result of the plane crashes in '80s and also he had economic problems, too," Sahebi told local television.
One witness, Jon Mohammad, a military pilot at Kabul Airport, told CNN that he jumped from a second floor window to the ground during the incident. He saw foreigners laying on the ground inside the first floor, he said.
"He was religious person, but I'm not sure if he had mental illness," Mohammad said of Gull, the pilot.
The shooting started at the Afghan national air force compound at North Kabul International Airport after an argument between the Afghan pilot and an international colleague, officials said. The NATO-led force said the Afghan military pilot opened fire on international trainers and a "gunfight" ensued.
"A 50-year-old man opened fire at armed U.S. military soldiers inside the airport after an argument between them turned serious," said Col. Baha Dur, chief of public relations for the Afghan National Army at Kabul military airport.
NATO said the confrontation took place at 10:25 a.m. at the airport, where a quick-reaction force responded to a "small arms fire incident." The airport is home to NATO Air Training Command Afghanistan.
An initial report indicates the attack occurred in a meeting room at the Afghan Air Force headquarters, a location operated and secured by Afghans, a U.S. military official told CNN. The NATO-led force is investigating, the official said.
Afghan President Hamid Karzai condemned the killings "by an Afghan military pilot."
Zaher Azimi, a spokesman for the Afghan Defense Ministry, said the killings upset Defense Minister Abdul Rahim Wardak and that "he shares the pain with the families of the victims."
Despite the account by international troops, a Taliban spokesman said a man named Azizullah was responsible.
"One suicide attacker ... managed to attack an Afghan military unit and has managed to kill many Afghan and international soldiers," Taliban spokesman Zabiullah Mujahid said.
The Taliban said the man killed nine foreigners and five Afghans before being killed by the Afghan army.
"We had worked hard on this plan for a long time," Mujahid told CNN. "He was cooperating with us since long time and he was providing us information about military air operations for a long time."
NATO disputed the Taliban claim.
"We do not know why it started but there is no indication that a suicide bomber was involved and there are no reports that someone managed to get into the base to do this," the NATO-led force said in a statement.
The Taliban has claimed responsibility for previous conflicts between NATO service members and members of the Afghan military. CNN could not independently verify the group's claims.
The Taliban said the man was once a pilot in an Afghan regime in the late 1980s and early 1990s.
"Since the current Afghan air forces have no planes, he was just going to Kabul airport to show up and earn his salary for a long time," Mujahid told CNN.
The man "was holding the rank of colonel at the time and he had an AK-47 with him. After his bullets were finished, then he was shot to death by armed forces," Mujahid told CNN.
There was confusion about the death toll. The NATO-led force initially said six service members were killed. It raised that toll to nine but backed away temporarily before saying again that the shooting killed nine people -- eight international service members and a civilian contractor. The Pentagon confirmed that all were Americans.
Later Wednesday NATO announced that two additional service members were killed in attacks elsewhere in the country, bringing the day's total number of NATO casualties to at least 11.
Violence between Afghan forces and NATO troops is a matter of extreme concern for NATO officials, and it is growing in frequency.
There have been 36 NATO deaths in the past two years attributed to attacks by people perceived to be Afghan soldiers or police. Officials fear that the increasing frequency of the attacks could undermine trust between NATO troops and the Afghans they are working hard to prepare so they can eventually take over security in the country.
The Taliban's claim that the Afghan gunman was their recruit follows a now-familiar pattern of the insurgency stating that attacks are theirs, even though NATO later suggests the gunman was acting out of personal motivation.
Out of 16 incidents of Afghan forces shooting NATO personnel that NATO has investigated, eight have been determined to be motivated by combat stress on the part of the Afghan attacker. The other eight investigations are undetermined.
CNN's Barbara Starr, Joe Sterling and Jennifer Rizzo contributed to this report