In this Thursday, May 24, 2012 photo provided by the U.S. Army, various military vehicles are seen parked after being cleaned and stripped of sensitive items for shipment as part of drawdown of 23,000 U.S. troops by Sept. 30, 2012 at the Kandahar Air Field south of Kabul, Afghanistan.

KABUL, Afghanistan — As the United States begins the major phase of its withdrawal from Afghanistan, military officials say equipment and vehicles are moving out of the country briskly but that planning the final details has been complicated because negotiations with the Afghan government have stalled over how many American troops might remain.

The complex push to get the equipment and vehicles out of land-locked Afghanistan and back to the U.S. is expected to cost up to $7 billion.

A year ago, the U.S. had about 50,000 vehicles in Afghanistan. About 25,000 are left, along with 20,000 shipping containers. About 1,200 damaged, worn-out or outmoded mine-resistant trucks will be chopped up and sold for scrap, and other vehicles will be loaned to partners in the NATO-led coalition here, turned over to Afghan forces or sold to friendly nations, said Brig. Gen. Duane A. Gamble, the deputy commander of the 1st Theater Sustainment Command in Kabul.

The coalition’s combat mission ends in December 2014. Senior U.S. commanders say they expect Afghanistan to sign an agreement calling for some U.S. troops to remain in the country as trainers and advisers, but negotiations over those remaining troops have been on hold for months now.

That’s forced logistics commanders to develop scenarios for what they’ll do if there’s no agreement and all U.S. forces are sent home, simply because they must have plans ready for all the possible situations.

An interim drawdown goal set by President Barack Obama is to reduce the 62,000 U.S. troops here to 34,000 by mid-February, and the departure of equipment and vehicles is essentially keeping pace with that of the troops, Gamble said.

The amount of materiel being shipped out has been relatively steady for months, but it’s expected to accelerate soon as the main part of that drawdown begins with the end of the summer fighting season.

It shouldn’t be a problem to handle that jump in volume, said Col. Jim Utley of U.S. Transportation Command, which oversees the logistics of getting people, vehicles and equipment to and from the United States and foreign operations.

“As the fighting season winds down and units start to redeploy, we’re sure the cargo pace will pick up, but I think we’ve built enough capacity that we can definitely meet the requirements, and we’re on a glide path to meet the presidential drawdown goals, so I think everything is going as well as could be expected,” Utley said.

“To be honest, up to this point, we haven’t really touched our capacity yet in terms of moving things out,” he said. “We built a lot of capacity; we have a lot of different routes, a lot of ways to move things out.”

The main route, via truck through Pakistan to the port of Karachi, for example, has about eight times the capacity that the Army currently is using, he said.

The U.S. has three basic ways to ship people and things out of Afghanistan. The cheapest by far is south through Pakistan. More expensive is the so-called northern distribution network, which is a host of convoluted routes across Afghanistan’s northern border through central Asia and various former Soviet bloc nations to several ports. The most expensive route is by air, either short hops to ports such as Dubai or directly back to the United States.

Sensitive cargo such as weapons and communication systems has to be flown, as does most equipment that belongs to units headed home, since they must refurbish it and get it ready in case they’re called to duty.

The military tries to ship most of the rest out via Pakistan to save money, but the Pakistani border is subject to frequent closings for reasons large and small.

After U.S. helicopters fired on a Pakistani border outpost in 2011, killing 24 Pakistani soldiers, that country closed the route for seven months. Earlier this year, road shipments out of Afghanistan were halted for several weeks when Afghan officials began charging customs duties. A few weeks ago, the route shut down for a week because of a port strike unrelated to the U.S. shipments.

Cultural differences also come into play. In July and August, the shipments by road were essentially halted for several weeks because truck drivers, security guards and border officials took time off for the Muslim holy month of Ramadan.

After that, the traffic through Pakistan, which for much of the year has carried about 70 percent of the materiel being shipped, resumed its normal pace.

The volatility of the route means it’s vital to keep the other options open, such as the routes to the north, by regularly shipping modest amounts of equipment through them, Utley said.

“We call it keeping it warm,” he said. “What we’re trying to do is maintain the entire network, and to do that we need to ship at least a minimal amount of cargo along those routes. That keeps the tactics, techniques and procedures fresh not only for our folks, but also keeps the officials working those locations engaged.”

One problem with using the northern routes as a full alternative is that some of the countries involved don’t allow shipments of U.S. combat vehicles. For now, only cargo in shipping containers goes that way.

If something unforeseen happens that prevents moving vehicles through Pakistan for an extended period, either they’d have to be flown out or the United States could try to negotiate their passage with the countries to the north, Gamble said.