AT A CLASSIFIED COMMANDO BASE, Afghanistan » One day this month, a pair of Russian Mi-17 assault helicopters delivered two teams of Afghan commandos, their faces obscured by black masks, in a touch-and-go landing at this camp in a lush valley encircled by frosty peaks about 50 miles from Kabul, Afghanistan.

A training squadron drawn from the most secretive counterterrorism units fielded by the United States and its NATO allies watched as the Afghan commandos stormed and cleared a three-story office building that was left conspicuously unfinished — the kind of structure favored by insurgents.

This is the combination of Afghan and allied troops that the Obama administration and the government in Kabul says will assume an increasing share of the combat burden in Afghanistan as the NATO alliance gradually hands over responsibility for security operations to Afghan troops.

As other troops are withdrawn, Special Operations forces are expected to make up almost one-third of the U.S. military presence in Afghanistan by next February. Their specialty — advising local military units on the front lines and hunting down top insurgent or terrorist leaders — will become the major focus of the alliance’s effort here until U.S. troops are either withdrawn by the target date of December 2014 or the Afghan government asks them to stay past then.

On any day in Afghanistan, there are about 60 Special Operations teams working with Afghan local police forces to provide security in villages; another 50 are assigned to Afghan strike forces, including nine commando battalions and special police units and 19 provincial response companies.

The most elite units are housed at secret bases like this one, where assault helicopters stand by to carry them on their missions. Other commando battalions and provincial response companies are scattered among population centers and along the ring road linking Afghanistan’s major cities.

For American commanders, the transition to Afghan leadership on security has been a challenge, requiring a sharp increase in the intensity of training.

It has also required a significant reorganization of planning — and a change in the culture of U.S. Special Operations forces, which, for the first time since the war began, answer to a single commander responsible for coordinating what had been separate, even conflicting, efforts.

"The dirty little secret among SOF is that we were competing among ourselves," said Maj. Gen. Tony Thomas, the senior commander overseeing all U.S. and allied Special Operations forces, or SOF, in Afghanistan.

"We didn’t necessarily share information to the greatest extent possible," said Thomas, an Army Ranger with a long career in Special Operations. "It wasn’t about who got the credit or glory — but we were all so focused on our individual mission that we didn’t always synchronize the effort in the most efficient way for a common goal."

There have been times when one strike team was targeting an insurgent suspect without knowing that a training team was courting his close kinsman to raise a local police force from their home village.

That began to change just under a year ago when Thomas took charge of a new military organization here — the Special Operations Joint Task Force-Afghanistan, making him, in essence, the first to lead a division-sized deployment of Special Operations forces. Under his command are all the various "tribes" of U.S. Special Operations forces: Army Green Berets, Navy SEALs and Marine Corps Special Operations units, as well as the top-tier strike teams that hunt down or kill high-value terrorist and insurgent leaders.

Senior officials say the new level of coordination has paid dividends in the form of initiatives like a centralized system for allocating drones, helicopters and airplanes that has allowed the 200 Special Operations aircraft to increase their sortie rate to 6,000 missions a month from 4,000.

Even as the number of U.S. troops will be cut in half from 68,000 by February 2014 under President Barack Obama’s withdrawal orders, the number of Special Operations forces will remain the same through the Afghan presidential election, which is scheduled for next spring, but could be delayed until closer to December 2014.

While the bulk of the U.S. and allied conventional forces remaining in Afghanistan will make the transition to a support role — and will be increasingly based at large military headquarters — the 10,000 U.S. Special Operations troops will continue to be deployed alongside Afghan units. (Including NATO and coalition troops, the total Special Operations deployment here numbers 13,700.)

"That partnering is rock solid, and we hope over time to come up off the tactical level," Thomas said. But he noted that Afghan and NATO leaders all understand the critical importance of assuring that next year’s elections are credible and secure. "So we’re probably going to stay a while longer at the tactical level than we were considering a year ago," he added.

Alliance commanders acknowledge that one of their greatest fears is an insurgent offensive on Kabul that, even if it fails, would so humiliate Afghanistan’s security forces that — like the Tet Offensive during the Vietnam War — it undermines support for the mission here and in nations that contribute troops to the effort.

Insurgents have mounted exactly that type of raid in the past. And while Afghan and NATO officials said at the time that local security forces reacted with speed and lethal professionalism to subdue the militants, the reality is different. Special Operations commanders now acknowledge that the responses were slow and clumsy and that Afghan troops needed to be strongly urged to move in.

"The Afghan army hadn’t performed that well in two previous tasks," said one NATO Special Operations commander here. "Both counterattacks had to be heavily mentored. It came out OK in the end — but only after a lot of prompting from our side."

The effort to sharpen the skills of Afghan Special Forces is focused at this base, which was opened to a reporter and photographer under the ground rules that its location and the Special Operations units operating here would not be disclosed. Here, the Afghan national commando strike force practices on a training range that includes a full-scale mock-up of a heroin laboratory, a fake village marketplace and that three-story, half-built office building.

The effort, according to allied and Afghan Special Operations commanders, is paying off: Twice in recent weeks, their units have prevented the use of truck bombs, each holding a charge several times larger than the explosives that leveled a federal building in Oklahoma City in 1995.

"The most important thing that this unit offers is security for Kabul, and as we move into this fighting season and into the upcoming elections in ’14, the population will view the security of the capital as a symbol of the credibility of their government," said Brig. Gen. James E. Kraft Jr., a deputy commander of the Special Operations Joint Task Force.

The commandos, he said, "have been making significant advances in securing the ‘rat lines’ leading toward Kabul and interdicting IED materials."

Shortcomings remain. One NATO Special Operations adviser said the training squadron is trying to "wean the Afghans off the ‘crack pipe’ of our aviation" — in particular troop transport and medical evacuation, which the Afghan forces cannot yet sustain on their own.

Although the unit based here is considered the most proficient in the country, allied officials said that it conducts 85 percent of its missions unilaterally, but still required coalition support for the other 15 percent.

© 2013 The New York Times Company