Updated 4:49 p.m. ET
KABUL, Afghanistan A member of the Afghan National Police opened fire on U.S. and Afghan forces at a police headquarters in eastern Afghanistan on Monday, sparking a firefight that killed two U.S. troops and two other Afghan policemen. The attacker was also killed in the shootout, officials said.
In a second incident, outside Kabul, U.S. troops fired on a truck approaching their military convoy, killing two Afghan men inside.
The shooting in the eastern Wardak province was the latest in a series of insider attacks against coalition and Afghan forces that have threatened to undermine their alliance at a time when cooperation would aid the planned handover of security responsibility to local forces next year.
The attack also comes a day after the expiration of the Afghan president's deadline for U.S. special forces to withdraw from the province.
U.S. officials have said that they are working with Afghan counterparts to answer President Hamid Karzai's concerns and maintain security in Wardak. Most of the U.S. troops in Wardak are special operations forces.
In Monday's attack, a member of the Afghan National Police uniform stood up in the back of a police pickup truck, grabbed a machine gun and started firing at the U.S. special operations forces and Afghan policemen in the police compound in Jalrez district, said the province's Deputy Police Chief Abdul Razaq Koraishi.
The assailant killed two Afghan policemen and wounded four, including the district police chief, before he was gunned down, Koraishi said.
There was some initial confusion whether the man was a member of the local police or the Afghan national police department.
The U.S. military said in a statement that two American service members were killed in the shooting.
U.S. forces were holding five Afghan police officers for questioning, Koraishi said.
Karzai ordered U.S. special operations forces to leave Wardak province, just outside the Afghan capital, because of that Afghans working with the U.S. commandos were involved in abusive behavior. Karzai gave them two weeks to leave, and the deadline expired Sunday.
On Sunday, Karzai accused U.S. forces of working with the Taliban to stage two suicide bombings over the weekend during the visit of U.S. Defense Secretary Chuck Hagel. In a speech, Karzai said the Americans want to scare Afghans into allowing them to stay.
That brought a sharp rebuke from the U.S. ambassador Tuesday, as news of the insider attack in Wardak emerged.
"The thought that we would collude with the Taliban flies in the face of everything we have done here and is absolutely without foundation," Ambassador James Cunningham said in a statement. "It is inconceivable that we would spend the lives of America's sons, daughters, and our treasure, in helping Afghans to secure and rebuild their country, and at the same time be engaged in endangering Afghanistan or its citizens."
Additionally, a Taliban spokesman denied that talks had resumed or that any progress had been made since they were halted in March of last year, reports CBS News correspondent Charlie D'Agata.
In the convoy shooting, U.S. forces spokesman Jamie Graybeal said the Afghan driver failed to heed instructions to stop as his truck came close to the American convoy near Kabul.
"The convoy took appropriate measures to protect themselves and engaged the vehicle, killing two individuals and injuring one," Graybeal said in an email. He said an assessment is underway.
Associated Press video shows a U.S. major cursing at one of his soldiers and slapping him over the head with his cap as Afghans pulled dead bodies from the truck. In the video, the major appears to be upbraiding the soldier for not using a laser warning device to signal the approaching truck.
The two dead men were employees of a company that repairs police vehicles, said Interior Ministry spokesman Sediq Sediqi. Another man was wounded in the shooting, said Col. Mohammad Alim, the police commander overseeing Kabul highways.