The deployment signals new resolve to crush militants — and hopefully capture fugitive leaders including Osama bin Laden — in an election year for both the Afghan and U.S. governments.
The U.S. Marines "have started to come into the country," said military spokesman Lt. Col. Bryan Hilferty. "Their deployment will take several weeks."
Hilferty said the Marines would be sent to one region of the country, where existing bases were being expanded to accommodate them. But he declined to say which area, or how long they would stay.
Meanwhile, in the latest U.S. operation, troops detained six suspected Taliban members — including a "mid-level" leader — in a raid in southern Afghanistan late Tuesday, Hilferty said. He did not disclose the leader's identity.
The extra Marines, deploying from ships in the Persian Gulf, will expand the U.S.-led force in Afghanistan to about 15,500, its biggest yet.
Many of the troops are strung across the lawless provinces near the Pakistani border as part of what U.S. commanders call a "hammer-and-anvil" strategy, drawn up with Pakistani forces on the other side.
The Marines are to adopt new U.S. tactics of sending forces from one unit to the same villages again and again in the hope of gaining better intelligence.
"They will conduct operations in one set area of the country to get to know the terrain and the local people," Hilferty said.
The military says the approach has helped them uncover weapons caches across the country, but it has yet to net any top militants.
The expanded U.S. force is also expected to help protect landmark Afghan elections from attacks by militants, including Taliban holdouts. Afghan President Hamid Karzai, who enjoys Washington's strong backing, is expected to win a five-year term if the September polls go ahead as planned.
The six suspected Taliban detained Tuesday were caught in a nighttime search operation near Deh Rawood, 250 miles southwest of the capital, Kabul, in Uruzgan province.
"We appear to have surprised the personnel there," Hilferty said. "They did not put up a fight." Troops found weapons and "other material which lead us to believe they are Taliban."
The province is considered a Taliban stronghold where some observers believe the hardline Islamic movement's supreme leader, Mullah Omar, could be hiding.
Thirteen people were killed in three days of fighting in Uruzgan earlier this month, including two U.S. soldiers and nine suspected militants.
More than 200 people have died in violence across Afghanistan so far this year, an unwelcome milestone as Karzai tries to convince donor nations gathered in Berlin to provide $27.5 billion for reconstruction.