Source: 9 killed in helicopter crash were Americans

Nine American service members died in a helicopter crash Tuesday in southern Afghanistan, where an international coalition has stepped up efforts against the Taliban, according to a Western defense source.

The source spoke on condition of anonymity when revealing the nationalities of the nine, who've added to the grim toll for Operation Enduring Freedom.

According to a CNN tally last updated Monday, 517 coalition troops had been killed in Afghanistan this year, including 341 Americans.

Even without the inclusion of the latest numbers, the figures make 2010 the deadliest year for Operation Enduring Freedom since it began nine years ago. (More: Home and Away: Casualties in Afghanistan and Iraq)

The International Security Assistance Force, as the coalition is known, did not release the nationalities of Tuesday's crash victims.

Two other service members, along with an Afghan National Army soldier and an American civilian, sustained injuries in the crash. They were taken to a NATO medical facility.

ISAF has ruled out enemy involvement in the crash, said Jan Rasuli, spokesman for Zabul Province's governor, Mohammad Jan Rasuli.

Earlier, the force had said that there were no reports of enemy fire in the area.

ISAF was formed under a U.N. mandate to bolster a secure environment and support the reconstruction of Afghanistan.

NATO took command in 2003.

The force comprises about troops from more than 40 countries, according to the organization.

U.S. President Barack Obama ordered the deployment of 30,000 extra U.S. forces to Afghanistan this year, increasing the total American troop commitment to almost 100,000, while at least 25 other countries pledged an additional 7,000 troops.

Unpopularity with the war in Afghanistan reached an all-time high in CNN polling in August, with 62 percent saying they oppose it. Moreover, confidence in the Afghan government is low. Seven in 10 Americans are not confident that Hamid Karzai's government can handle the situation there.

The Obama administration says it will begin withdrawing troops from Afghanistan in August 2011, depending on conditions on the ground.

CNN's Atia Abawi and Matiullah Mati contributed to this report.