CAMP LEATHERNECK, Afghanistan -- The surge of U.S. Marines and other forces in southwestern Afghanistan has broken the Taliban's grip on a former stronghold, allowing coalition forces to begin turning over security to Afghans, commanders said.

"It will be a rolling transition to more training and advising and assisting and less of the counterinsurgency operations," Gen. James Amos, commandant of the Marine Corps, told USA Today during a short visit to Afghanistan. "The Afghan security forces will be in the lead and we'll be in support."

The transition comes as the United States plans to withdraw 23,000 troops from Afghanistan by the end of September, leaving about 68,000 U.S. troops nationwide. The pullout was ordered by President Obama, who wants most combat forces out of Afghanistan by the end of 2014.

Obama ordered a troop surge of 33,000 more than two years ago to reverse gains made by the Taliban. Many of those initial forces were Marines sent to Helmand province, a former Taliban stronghold and the world's leading poppy growing region.

The number of U.S. Marines in the Helmand region has declined to 13,000 from a peak of 21,000 last year. At the same time, the size of Afghan security forces in the region has grown to 15,000 soldiers and about 8,000 police.

The gradual shift of security duties in Helmand to Afghan forces will have implications nationwide, analysts said.

"This is a big test case for the future of Afghanistan," said Seth Jones, an analyst at the RAND think tank.

The hulking old tanks, left to rust when Soviet forces pulled out of Afghanistan in 1989, still packed a threat when Albert Whittington arrived.

Whittington, an ordnance and explosives specialist with the Baltimore district of the Army Corps of Engineers, clambered through dozens of Red Army tanks, trench-digging vehicles, bridge-laying equipment and other derelict machinery at the Pul-e-Charki military base east of Kabul.

His mission: Find any unexploded ordnance, unused ammunition and other materials still capable of maiming or killing.

Whittington, 39, has spent the past year working to make Afghanistan safer from one of the deadly legacies of war.

More than three decades of conflict in Afghanistan — from the Soviet invasion of 1979 through the civil war of the 1990s to the U.S.-led invasion after the terrorist attacks of Sept. 11, 2001 — have left the impoverished farming country one of the most heavily mined in the world.

"Basically, Afghanistan has been contaminated with unexploded ordnance by the full range of actors," said Elena Rice, a program officer with the United Nations Mine Action Service.

Whittington, an Army veteran who lives in LaPlata, has seen material in Afghanistan from "most of the major countries" — Britain, Russia, China, the United States — and many smaller nations.

They have left the country with 6,000 known hazardous areas, danger zones that disrupt farming, housing, resettlement and development in 1,900 communities across Afghanistan, according to the U.N. agency.

Speaking by telephone from the capital, Kabul, Whittington described a typical explosion.

"Kids, they'll be out playing ball and they'll come across something neat and shiny," he said. "They go over to play with it, and the next thing you know, they drop it and it goes boom — and you have children that are injured, or worse."

The United States, which began working in the 1990s to clear Afghanistan of the explosive remnants of war, is the largest donor in the international effort to eliminate the threat within a decade.

The U.S., the United Nations and others have paid to train 14,000 Afghans to identify, safely handle and dispose of unexploded ordnance, Rice says. If there is continued funding, she says, the work is on schedule for completion in 10 years.

For now, the weapons continue to kill. An average of 33 people per month were injured or killed in Afghanistan last year by such munitions. More than half the victims were boys ages 7 to 14.