Three Americans and two other international troops were killed Friday in an attack in eastern Afghanistan, officials said.

Insurgents attacked Afghan and international forces Friday with rocket-propelled grenades and guns, NATO forces said in a statement. The troops called in air support, forcing the militants to withdraw. They are being pursued, the statement said.

Col. Greg Julian, a spokesman for U.S. Forces in Afghanistan, confirmed that three of the dead were Americans. The nationalities of the other two were not immediately known because NATO typically waits for countries to release such information for their own soldiers.

Meanwhile, in a political development related to the country's upcoming presidential election, Afghanistan's top vice president broke away from the president to join a competing ticket, a spokesman said.

Former warlord Gul Agha Sherzai has appointed First Vice President Ahmad Zia Masood as his top deputy in his run against President Hamid Karzai, a Sherzai spokesman said.

The spokesman, Gul Khalid Pushtoon, said Sherzai, now governor of eastern Nangarhar province, plans to file official papers on Saturday.

Government spokesman Waheed Omar said he had not been informed of Masood's decision but that the government would not stand in his way.

"Masood is still the vice president of Afghanistan, but if he has decided that joining a different team will help, I don't think the president will have a problem because that is his constitutional right," he said.

Spokesmen for Masood could not immediately be reached for comment.

Sherzai has been a strong supporter of Karzai but Pushtoon said they are running against the president because they don't believe he is popular enough with the citizenry to hold onto the job.

"We think that President Karzai will not win the election, so therefore we want to keep the leadership in the region as well as improve the economy of the country," Pushtoon said.