Afghanistan-bound NATO oil tankers, were set on fire by alleged militants at the Pakistani border post of Torkham in March. (Qazi Rauf/Associated Press)

The U.S. military is rapidly expanding its aerial and Central Asian supply routes to the war in Afghanistan, fearing that Pakistan could cut off the main means of providing American and NATO forces with fuel, food and equipment.

Although Pakistan has not explicitly threatened to sever the supply lines, Pentagon officials said they are concerned the routes could be endangered by the deterioration of U.S.-Pakistan relations, partly fed by ill will from the cross-border raid that killed Osama bin Laden.

Memories are fresh of Pakistan’s temporary closure of a major crossing into Afghanistan in September, resulting in a logjam of hundreds of supply trucks and fuel tankers, dozens of which were destroyed in attacks by insurgents.

While reducing the shipment of cargo through Pakistan would address a strategic weakness that U.S. military officials have long considered an Achilles’ heel, shifting supply lines elsewhere would substantially increase the cost of the war and make the United States more dependent on authoritarian countries in Central Asia.

A senior U.S. defense official said the military wants to keep using Pakistan, which offers the most direct and the cheapest routes to Afghanistan. But the Pentagon also wants the ability to bypass the country if necessary.

With landlocked Afghanistan lacking seaports, and hostile Iran blocking access from the west, Pentagon logisticians have limited alternatives.

“It’s either Central Asia or Pakistan — those are the two choices. We’d like to have both,” the defense official said, speaking on the condition of anonymity to avoid alienating Pakistan. “We’d like to have a balance between them, and not be dependent on either one, but always have the possibility of switching.”

U.S. military officials said they have emergency backup plans in case the Pakistan routes became unavailable.

“We will be on time, all the time,” said Vice Adm. Mark D. Harnitchek, deputy commander of the U.S. Transportation Command, which oversees the movement of supplies and equipment.

In such an event, however, the military would have to deliver the bulk of its cargo by air, a method that might not be sustainable; it costs up to 10 times as much as shipping via Pakistan.

“We’d have to be a little bit more mindful of what we put in the pipe,” Harnitchek said.

The Defense Department is already boosting the amount of cargo it sends to Afghanistan by air. To save on costs, the military is shipping as many of those supplies as possible to seaports in the Persian Gulf before loading them on planes bound for the war zone.

As recently as 2009, the U.S. military moved 90 percent of its surface cargo through Pakistan, arriving by ship at the port in Karachi and then snaking through mountain passes, deserts and remote tribal areas before crossing the border into Afghanistan. The Pakistan supply lines are served entirely by contractors instead of U.S. military convoys and are vulnerable to bandits, insurgents and natural disasters.

Today, almost 40 percent of surface cargo arrives in Afghanistan from the north, along a patchwork of Central Asian rail and road routes that the Pentagon calls the Northern Distribution Network. Military planners said they are pushing to raise the northern network’s share to as much as 75 percent by the end of this year.

Obama administration officials said they are negotiating expanded agreements with Kazakhstan, Uzbekistan and other countries that would allow for the delivery of additional supplies to the Afghan war zone. Washington also wants permission to withdraw vehicles and other equipment from Afghanistan as the U.S. military prepares to pull out one-third of its forces by September 2012.

By shifting the burden to Central Asia, however, the U.S. military has become increasingly reliant on authoritarian countries, prompting criticism from human rights groups that the Obama administration is cozying up to dictators.

For instance, more than one-third of the northern-route cargo passes through tiny Azerbaijan, a country saddled by “pervasive corruption,” according to the State Department’s annual human rights report. U.S. defense officials also say the northern supply lines would not be possible without the cooperation of Russia. One new route runs through Siberia.

The biggest potential choke point, however, lies in Uzbekistan, a former Soviet republic that borders northern Afghanistan. It previously had kicked the U.S. military out of the country after Washington complained about the killing of hundreds of protesters in 2005.

But as the United States has deepened its involvement in Afghanistan, relations with Uzbekistan have warmed up again. Today, more than 80 percent of supplies shipped along the Northern Distribution Network pass through the country.

Expanded supply lines

The northern routes were developed in the waning days of the George W. Bush administration. Since then, the U.S. government has expanded the network into a spiderweb of supply lines.

Some start at Baltic seaports and run through Russia and Central Asia by rail. Another key line picks up traffic on the Black Sea and funnels it through the Caucasus region. One winding truck route begins at a U.S. Army depot at Germersheim, Germany, and ends, an average of 60 days later, at Bagram air base in Afghanistan. As with the Pakistan routes, the deliveries are all made by contractors.

“If you look at what we’ve done there in the last two years, we look at it more or less as a logistics miracle,” said Alan F. Estevez, the Pentagon’s principal deputy assistant secretary for logistics.

There are two big limitations, however, on what the Pentagon can ship through Central Asia. First, supplies are generally restricted to food, water and construction material; ammunition, weapons and other “lethal” cargo are prohibited.

Also, the routes are strictly one-way. Nothing can be shipped back out of the war zone.

U.S. officials said they are trying to negotiate deals with several countries to remove those restrictions. That will be crucial as the United States withdraws 33,000 troops from Afghanistan over the next 15 months, military leaders said.

Perhaps the most vital section in the northern network is a rail line that crosses south through Uzbekistan and over the Amu Darya river to reach Hairaton, Afghanistan. About five out of every six cargo containers travel this route.

“In reality, Uzbekistan is really at the center of all these routes,” said Alexander Cooley, a Barnard College professor and an expert on U.S. military relations in Central Asia. “They’re certainly in the catbird seat. And they know it.”

The final leg of the Uzbek rail line, from the city of Karshi to the Afghan border, underscores how the U.S. military has been forced to rely on rickety routes to sustain its troops.

In November 2009, U.S. embassy officials in Tashkent, the Uzbek capital, were warned by a confidential source that the tracks were brittle and at risk of fracturing if trains carried more than half their usual loads. On top of that, the Soviet-era locomotives carrying U.S. cargo were not designed to cross steep mountains; engineers had to apply the brakes almost constantly as they moved downhill.

“By the time the trains have descended from the mountains, the wheels are glowing red hot,” the embassy reported in a diplomatic cable. The source, an engineer, said he was “appalled by how long it takes to transport anything by rail in Uzbekistan” and that he refused to take the train for fear of a crash.

The cable, titled “Uzbek Rail: Red Hot Wheels to Afghanistan” and obtained by the anti-secrecy Web site WikiLeaks, concluded that “a train wreck is possible in the literal sense.”

U.S. military officials said they knew of no accidents or safety problems on the 200-mile rail segment. In February, Uzbekistan announced it had obtained a $218 million loan from Japan to upgrade the line to the Afghan border.

Human rights concerns

Uzbekistan has been assailed by human rights groups for repression under President Islam Karimov, who has ruled the country since the breakup of the Soviet Union. Freedom House, a Washington-based advocacy group, ranks it as one of the nine worst countries in the world for civil liberties and political rights.

From 2001 to 2005, the U.S. military relied on an Uzbek air base as a hub for combat and supply missions to Afghanistan. U.S. forces were evicted from the base after Washington pressured Karimov to allow an international probe into the deaths of hundreds of anti-government protesters in the province of Andijan.

Since 2008, however, Washington has steadily worked to repair relations. A stream of U.S. military leaders and diplomats has visited Tashkent, including Secretary of State Hillary Rodham Clinton in December and Denis McDonough, the deputy national security adviser, in late May. Uzbekistan, in turn, has reopened its railroads, highways and airspace for U.S. cargo.

Thomas M. Sanderson, an analyst with the Center for Strategic and International Studies, said that the Obama administration has continued to raise human rights concerns with Uzbekistan but that the Afghan supply routes usually take precedence.

“There is no doubt about it. We are there for one primary reason, and that is to enable our operations in Afghanistan,” said Sanderson, who has studied the Northern Distribution Network.

State Department officials said they do not hesitate to press Uzbekistan to improve its human rights record. When Clinton visited Tashkent, they noted, she made a point of meeting activists and calling for the release of jailed journalists.

“We’ve made a real effort to try to engage Uzbekistan on human rights and in trafficking persons, and in some cases there’s been some progress,” said Robert O. Blake, assistant secretary of state for South and Central Asia. “This is something that’s in their own interest to do, to allow greater freedom of religion and greater freedom of expression.”

Diplomatic cables, however, show Uzbek officials have not hesitated to demand U.S. restraint on human rights in exchange for cooperation on the supply routes.

In March 2009, shortly after the State Department gave an award to an Uzbek human rights activist, Foreign Minister Vladi­mir Norov made an “implicit threat” to suspend deliveries to Afghanistan, according to a cable signed by Richard B. Norland, the U.S. ambassador in Tashkent at the time.

An angry Karimov also complained to Norland personally.

“Put yourself in my place,” Karimov told the ambassador, according to the cable. “Would you trust me if I had done this?”

In that cable and others to Washington, Norland counseled the Obama administration to check its public criticism of Karimov to maintain the viability of the supply lines. In advance of a visit to Tashkent by a senior State Department official, Norland advised using “private, but frank diplomacy” to cajole Uzbekistan rather than “more openly coercive measures.”

“Uzbek pride often gets the better of rationality and officials here will think nothing of cutting off their nose to spite their face,” Norland added in a July 2009 cable.