By Yeganeh Torbati

KABUL (Reuters) - The senior U.S. commander in Afghanistan will have greater freedom to strike at the Taliban under broad new powers approved last month by President Barack Obama, U.S. Defense Secretary Ash Carter said on Tuesday.

Carter, on an unannounced visit to Afghanistan, said the powers granted General John Nicholson would allow "much more efficient use and effective use of the forces we have here as well as the Afghan forces."

The new authorities give U.S. forces greater ability to accompany and enable Afghan forces battling a resilient Taliban insurgency, while also allowing greater use of U.S. air power, particularly close air support. Previously, Nicholson - who commands both the NATO-led Resolute Support mission and a separate U.S. counter terrorism mission - was only allowed to take action against the Taliban "in extremis," or moments when assistance was needed to prevent a significant Afghan military setback.

"Whereas previously he waited until a situation had developed in which Afghan forces really needed our enabling support, now he's able to look ahead," Carter said at a talk with American troops at Bagram air base, flanked by Nicholson.

"That's a better way of making use of what we have here now," he said.

Afghan forces, fighting largely on their own since the NATO-led mission ended most combat operations in 2014, have frequently asked for more combat assistance from their allies, particularly for close air strikes.

Nicholson said the broader authorities were being used "almost daily" in support of Afghan forces. He pointed to a significant pickup in the pace of U.S. operations 18 months after the end of the main NATO combat mission.

A U.S. military official said the new authorities had been used a couple of dozen times since they were approved by the White House in June.

Carter's visit comes days after Obama shelved plans to cut the U.S. force in Afghanistan nearly in half by year's end, opting instead to keep 8,400 troops there through to the end of his presidency in January.

Of that number, around 3,000 will be advising Afghan troops, and 3,300 will be working as "enablers" in support roles, Nicholson said. An additional 400 troops based "over the horizon" - outside Afghanistan - will round out the U.S. contribution to the NATO mission. An additional 2,150 U.S. troops will be focused on counter terrorism operations, he said.

As an example of how the new authorities are being used, Nicholson cited "combat enablers" the United States had provided to the Afghans in their fight to expand the territory they control around the northern city of Kunduz. Such enablers can serve in a broad variety of roles, such as engineers or electronic warfare specialists.

He contrasted that with the U.S. role last year, when Kunduz was briefly captured by the Taliban. At that time, the city was defended by Afghan forces battling largely without NATO's support.

"It's more of an offensive nature to that operation that we're assisting," Nicholson told reporters in Bagram.

Americans have been backing up Afghan troops fighting Taliban militants in other hotly contested regions, including Uruzgan and Maiwand in Kandahar province in the south.

"I think all the uses have been right in line with the intended purpose that the president gave us," Nicholson said.

The Taliban have made major gains and are estimated to control more territory than at any time since they were driven from power by a U.S.-led campaign in 2001 aimed at dismantling al Qaeda and denying it safe haven in Afghanistan.

But after a difficult year in 2015, when the insurgents briefly captured Kunduz, Afghan and international officials say government troops have succeeded in stabilizing the situation to a large extent.

Carter, who met both Ghani and Chief Executive Abdullah Abdullah as well as senior U.S. military commanders, said it was "critical" that the national unity government formed after disputed elections in 2014 maintained stability.

Ghani thanked the United States as well as other NATO allies who last week pledged to maintain support for Afghanistan. He also praised Afghan forces, who he said had been "standing tall" since the departure of foreign combat forces.

(Reporting by Yeganeh Torbati; editing by Mike Collett-White and Andrew Hay)