"The operation is part of an overall aim to deny the al Qaeda, (and) Taliban freedom of movement, resupply in Khost region," Marine Lt. Col. Ben Curry said.
The Marines, trained for mountain warfare, have carried out three operations since they arrived in Afghanistan in April but have not found any al Qaeda or Taliban fighters.
"There isn't a quick fix, this is going to be a more sustained operation running into weeks," Curry said at Bagram air base near Kabul.
Both Khost and neighboring Paktia province have been the focus of coalition activity and concern, because remnants of Taliban and al Qaeda forces are believed hiding there. Clashes between rival militia groups in the region have also helped render the area unstable.
Officials warned of attempted car bombings and suicide attacks by militants to disrupt the Loya Jirga, or traditional grand assembly, due to begin in Kabul on June 10. The meeting, seen as a key step in forming a stable government in the war-torn country, will result in an interim administration.
Another tactic for the enemy would be to score a "symbolic victory" against the coalition. Shooting down a helicopter or capturing a soldier and executing him would give heart to anti-Western forces, a senior British military official said.
In another development, three rockets were fired at an allied military camp in eastern Afghanistan but no one was hurt and no damage was caused, a Pakistan-based Afghan news service said on Thursday. The Afghan Islamic Press reported that unidentified people fired the rockets at the camp, about three miles east of the provincial capital Gardez, on Wednesday night.
The operation near Afghanistan's border with Pakistan comes as concern has mounted that the hunt for the Islamic militants might suffer as a result of rising tension between Pakistan and India.
Pakistan has said it might move troops from its western border with Afghanistan to its eastern border where it has been confronting Indian forces. An attack on an Indian army camp in disputed Kashmir this month escalated tensions between India and Pakistan.
The fundamentalist Taliban, ousted from power in December after a U.S.-led air campaign and opposition advances, and al Qaeda, blamed for the Sept. 11 attacks on the United States, have not been seen in large numbers since March, when U.S.-led troops took on several hundred in the last big battle of the Afghan war.
Earlier this month, a 1,000-strong international force returned from a sweep of mountains north of Khost without encountering any al Qaeda or Taliban.
With all the military activity, the tide of refugees returning to Afghanistan has not slowed. The number of refugees returning since March has already exceeded the total expected for the entire year. The United Nations says it will have to turn to donors for more money.
The U.N.'s High Commissioner for Refugees said 798,000 Afghans had returned home by Wednesday night and the numbers were expected to reach 810,000 by Thursday nightfall, while its budgeted target for the whole of 2002 was for 800,000.