U.S. Marines and Afghan troops launched an offensive Saturday to seize control of a remote mountain valley from insurgents tied to the deadliest attack on American forces since the Taliban was ousted nearly four years ago.

The operation is the biggest yet aimed at rebels believed to be responsible for twin attacks that killed 19 U.S. troops in June. Three Navy SEALs were killed in an ambush, and all 16 soldiers on a helicopter sent to rescue them died when it was hit by a rocket-propelled grenade.

The offensive came at the end of a deadly week in Afghanistan. Seven Americans were killed along with dozens of rebels and civilians, reinforcing concerns that crucial legislative elections next month could be threatened by a surge in violence.

U.S. and Afghan commanders said fighters in the Korengal Valley, in northeastern Konar province near the Pakistani border, were intent on disrupting voting. They said hundreds of Afghan rebels, as well as extremists from Pakistan, Saudi Arabia and Chechnya, were operating in the valley.

"We want them running for their lives way up in the hills where they can't attack polling stations," said Capt. John Moshane of the 2nd Battalion, 3rd Marine Regiment, based in Hawaii. "We want to isolate them from the community."

Hundreds of Marines and Afghan special forces troopers started moving into position at one end of the valley Thursday, about 120 miles east of the capital, Kabul. They dug mortar and machine-gun pits for a resupply base in a corn field near Kandagal, a village of about 100 farm families.

Reacting quickly, rebels fired rockets at a nearby U.S. post and a troop convoy but did not hit anything.

U.S. and Afghan forces hiked into the rugged mountains Friday and Saturday, many leading lines of donkeys laden with food and water. A-10 attack planes circled high above. The operation was expected to last at least two weeks, Moshane said.

One of the main objectives of the offensive is breaking up a network of fighters led by a Taliban officer, Ahmad Shah, also known as Ismail, who asserted responsibility for the June attacks, said Kirimat Tanhah, a commander in the U.S.-trained and financed Afghan special forces. Shah is suspected of having ties to al Qaeda fighters in Pakistan, he said.

Lt. Col. Jim Donnellan, commander of the Marine battalion, said the valley was a base for lots of other "bad guys," including al Qaeda fighters, allies of a renegade former minister, Gulbuddin Hekmatyar, other Taliban groups and dozens of criminals involved in timber and gem smuggling.

Meanwhile, a shepherd who rescued the only member of the ambushed SEAL team to survive the June 28 ambush was reported to be in hiding after extremists threatened to kill him.

Donnellan confirmed that the fourth SEAL was sheltered by a villager but declined to elaborate. He did not comment on the accounts of tribesmen who said the shepherd, Sher Alam, was hiding from extremists.

"Men distributed leaflets around our village saying they were going to kill him," said Shah Wali, a neighbor. "His wife and children are being protected by others in the village, but Sher had to leave."

U.S. Marines load mortar shells into containers as they set up positions in Kandagal, in east Afghanistan, to attack militants suspected of ambushing U.S. troops.