Afghan insurgents shot down a U.S. transport helicopter in Wardak province early Saturday killing 30 U.S. troops, seven Afghan commandos and one civilian interpreter.
The incident represents the highest number of U.S. forces killed during a single event in support of Operation Enduring Freedom.
The Taliban took the responsibility of the attack on the CH-47 Chinook with a rocket-propelled grenade.
Some of the dead Americans belonged to the elite force SEAL Team 6, which killed the most wanted terrorist Osama bin Laden in May in Abottabad in Pakistan.
NATO-led International Security Assistant Force (ISAF) confirmed the report of the crash but did not reveal the cause. "An investigation is underway to determine the exact cause of the crash. Additional details will be provided as they become available," it said.
American President Barak Obama offered condolences to the families of 30 U.S. soldiers and their Afghan counterparts killed in the crash.
"My thoughts and prayers go out to the families and loved ones of the Americans who were lost earlier today in Afghanistan," Obama said in a statement.
"Their deaths are a reminder of the extraordinary sacrifices made by the men and women of our military and their families, including all who have served in Afghanistan."
Secretary of Defense Leon Panetta also mourned the deaths and said: "I am deeply saddened by the loss of many outstanding Americans in uniform and of their Afghan counterparts earlier today in Afghanistan. Their courage was exemplary, as was their determination to make this a safer world for their countries and for their fellow citizens."
"We will stay the course to complete that mission, for which they and all who have served and lost their lives in Afghanistan have made the ultimate sacrifice. They and their families are in my thoughts, in my prayers and in my heart," Panetta added.
Commander, ISAF, General John R. Allen who suffered a jolt due to the incident said, "No words describe the sorrow we feel in the wake of this tragic loss."
"All of those killed in this operation were true heroes who had already given so much in the defense of freedom. Their sacrifice will not be forgotten. Our thoughts and prayers are with the families who are now waiting for their loved ones to return home. We will do everything in our power to support them in this time of need."
"We also mourn the loss of our heroic Afghan partners who fight with us shoulder to shoulder, every day," added Allen.
In year 2005, two CH-47 Chinook crashed, killing 32 U.S. servicemen and three civilian contractors in total.
According to icasuality.org data, in 2011 only 379 soldiers have lost their lives in Operation Enduring Freedom. Out of 379, the U.S. lost 282 of its soldiers this year. Since the start of Operation Enduring Freedom in 2001 the U.S. has lost 1,728 soldiers and the number is very high in comparison with its ally UK which lost 380 soldiers in the whole operation in 10 years.
Operation Enduring Freedom has claimed the lives of 2,660 foreign soldiers so far.