U.S. Marines and Afghan troops launched an offensive Saturday to take a remote mountain valley from insurgents tied to the deadliest blow on American forces since the Taliban (search) regime was ousted nearly four years ago.

The operation is the biggest yet aimed at rebels believed responsible for twin attacks that killed 19 U.S. troops in June. Three Navy SEALs (search) 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 for U.S. forces in Afghanistan. Seven Americans have died along with dozens of militants and civilians, reinforcing concerns that crucial legislative elections next month could be threatened by a surge in violence.

U.S. and Afghan commanders said militants in the Korengal Valley (search), in eastern Kunar province near the Pakistani border, were intent on disrupting voting. They said the valley held hundreds of Afghan rebels, as well as extremists from Pakistan, Saudi Arabia and Chechnya.

"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.

American 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 is breaking up a network of militants led by a local Taliban officer, Ahmad Shah, also known as Ismail, who claimed responsibility for the June 28 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 militants in Pakistan, he said.

"Ismail's men ambushed the SEAL team and shot down the helicopter," Tanhah told The Associated Press. "Many of them are foreigners and have trained in Pakistan and elsewhere."

He said Shah also pays impoverished villagers to fight for him.

Lt. Col. Jim Donnellan, commander of the Marine battalion, said the valley was a base for lots of other "bad guys" besides Shah, including Al Qaeda militants, fighters loyal to renegade former premier Gulbuddin Hekmatyar (search) and other Taliban groups.

Dozens of criminals involved in timber and gem smuggling are there, too, he said.

"Some of them are thugs, others are political ideologues, coming in and throwing their money around," Donnellan said. "Many villagers are paid good money to work with the militants."

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

Donnellan, the Marine commander, confirmed the fourth SEAL was sheltered by a villager, but he declined to elaborate. He didn't comment on the accounts of tribesmen who said 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."

He said Alam was grazing his animals when he found the wounded commando hiding in the mountains after the ambush. Wali said the SEAL pointed his gun at Alam, but the shepherd raised his shirt to show he had no weapon and was not a threat.

Alam took the man to his home and bandaged his wounds, before walking to a nearby U.S. base to alert them, Wali said.

He said Alam, who is Pashtun, the same ethnicity as most Taliban fighters, gave sanctuary to the American because "it is our culture."

"We would help anyone who asks, anyone ... well, except Usama bin Laden because he damaged our country," Wali added.