Nearly two dozen members of SEAL Team 6 were among 30 American troops killed Saturday when the Taliban shot down their helicopter during an overnight operation in eastern Afghanistan.

Their mission was to sweep into Wardak Province at night, and take down a suspected Taliban compound. But just as they arrived, the helicopter exploded. Officials say they believe a rocket-propelled grenade slammed into the helicopter, killing all on board.

The majority of the dead were part of Team 6, the commando unit that killed Osama bin Laden in May, though officials have said none of the SEALs killed in the crash took part in the bin Laden raid.

The incident marked America's highest loss of life since June 2005, when another Chinook helicopter, again brought down by Taliban fire, killed 16 service members that day; most of them were also Navy SEALs.

Now that the southern provinces of Helmand and Kandhar have become more tame through the so-called "surge" in U.S. troops, eastern Afghanistan has become the toughest fighting ground for American troops, as it is easy for Taliban fighters to slip across the border from their Pakistani safe havens to hide in the mountains for cover.

"All of those killed in the operation were true heroes who had already given so much in the defense of freedom," said Gen. John Allen, the top commander in Afghanistan.

President Obama said the tragedy reflects the "extraordinary sacrifices" made by the military and their families.

And Adm. Mike Mullen, chairman of the Joint Chiefs of Staff, said despite the tragedy, the American military operation must go on.

"The best way we can honor that sacrifice, is to keep at it, keep fighting, keep moving forward," Mullen said.