BAGRAM AIR FIELD, Afghanistan — Speaking to nearly 4,000 U.S. troops here after a secret overnight flight from Washington, President Barack Obama on Friday said they were making "important progress" against militants in Afghanistan.

"Today we can be proud that there are fewer areas under Taliban control," he said.

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"We said we were going to break the Taliban's momentum," he added. "That's what you're doing. You're going on the offense, tired of playing defense."

Obama had traveled to Afghanistan to thank the troops and to deal with frayed relations with Afghan President Hamid Karzai.

But plans for a face-to-face meeting with Karzai were abruptly scrapped. The White House and Afghan officials blamed the last-minute cancellation on rough weather. Instead, the two leaders spoke by phone, Obama at the air base and Karzai in Kabul.

Obama's surprise visit to the war zone, his second as president, came 10 days before he is to address the nation about a new review of U.S. strategy to defeat the Taliban and strengthen the Afghan government so American troops can begin leaving next year.

The trip also came at a particularly awkward moment in already strained U.S. relations with Afghanistan because of new and embarrassing leaked cables alleging widespread fraud and underscoring deep American concerns about Karzai.

There was no mention of that as the president spoke to more than 3,500 service members packed into a huge airplane hangar. After his remarks, he spent more than 10 minutes shaking hands, going around the hangar three times as they grabbed his hand and held cameras and cell phones high to take photos.

Jim Young  /  Reuters

President Barack Obama greets troops at Bagram Air Base in Afghanistan on Friday.

Obama stayed on this U.S. military base, the headquarters of the 101st Airborne Division, the entire time he was here, just under four hours. He huddled with U.S. Gen. David Petraeus, the top NATO commander in Afghanistan and U.S. Ambassador Karl Eikenberry. And he visited wounded soldiers at a base hospital, personally dispensing five Purple Hearts to wounded service members.

"Because of the progress we're making, we look forward to a new phase next year, the beginning of the transition to Afghan responsibility," Obama told the troops. He thanked them for their efforts, noting the difficulty in being away from home during the holidays, and they repeatedly cheered him in return.

He said the U.S. was continuing "to forge a partnership with the Afghan people for the long term." And he said, "we will never let this country serve as a safe haven for terrorists who would attack the United States of America again. That will never happen."

There are now about 150,000 coalition forces in Afghanistan, roughly 100,000 of them Americans. The U.S. and its NATO partners agreed last month in Lisbon, Portugal, to begin turning over control to local Afghan authorities in 2011, with a goal of completing that transition by the end of 2014.

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White House officials said gusty winds and swirling dust led them to cancel Obama's planned helicopter visit to Kabul, about 30 miles north of here. A backup plan for a secure videoconference was also scrapped.

Waheed Omar, a Karzai spokesman, said the Afghan leader was "not upset" that the palace visit was scuttled. He noted that the two leaders had met during the conference in Lisbon and discussed the situation in Afghanistan in detail.

Obama, who has tripled U.S. troop strength in Afghanistan, has come under increasing pressure to demonstrate progress in turning the tide against the Taliban insurgency in the battle that has now gone on for more than nine years. In his remarks to the troops, Obama cited "important progress."

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"We said we were going to break the Taliban's momentum. And that's what you're doing. You're going on the offense, tired of playing defense, targeting their leaders, pushing them out of their strongholds. Today, we can be proud that there are fewer areas under Taliban control and more Afghans have a chance to build a more hopeful future," he said.

He thanked the troops for their work and sacrifice "on behalf of more than 300 million Americans."

"You give me hope. You give me inspiration. Your resolve shows that Americans will never succumb to fear," he said to cheers and shouts.

Petraeus, the commander Obama is looking to to turn things around, introduced Obama to the troops and teased the president about the basketball injury to his lip last week. Presenting him with a 101st Airborne T-shirt, Petraeus told the president: "No one will mess with you if you wear this, Mr. President."

At the base hospital, Obama met with platoon members from the unit that lost six soldiers this week in brazen killings by an Afghan border policeman who turned fire on his U.S. trainers.

Mentioning that visit and his meeting with what Petraeus called "wounded warriors," Obama told the assembled troops: "I don't need to tell you this is a tough fight. ... It's a tough business. Progress comes slow. And there are going to be difficult days ahead. Progress comes at a high price."

Newly leaked U.S. cables show American diplomats portraying Afghanistan as rife with graft to the highest levels of government, with tens of millions of dollars flowing out of the country and a cash transfer network that facilitates bribes for corrupt Afghan officials, drug traffickers and insurgents.

A main concern in the cables appears to be Karzai himself, who emerges as a mercurial figure. In a July 7, 2009, dispatch, Eikenberry describes "two contrasting portraits" of the Afghan president.

"The first is of a paranoid and weak individual unfamiliar with the basics of nation building and overly self-conscious that his time in the spotlight of glowing reviews from the international community has passed," the cable says. "The other is that of an ever-shrewd politician who sees himself as a nationalist hero. ... In order to recalibrate our relationship with Karzai, we must deal with and challenge both of these personalities."

Obama aides later said the subject of the cables didn't come up during the Obama-Karzai phone call, which lasted 15 minutes. Ben Rhodes, a White House national security aide, told reporters Secretary of State Hillary Rodham Clinton had already spoken to Karzai about WikiLeaks disclosures.

After the long, unannounced flight from Washington, Obama landed in darkness under intense security.

He stepped off Air Force One clad in a brown leather jacket that he was also wearing when he spoke to troops. Plans of his trip into the war zone were tightly guarded.

Despite the upcoming review results, White House officials on the trip played down the significance of his upcoming speech. No big policy changes are expected, they said.

To deal with any doubts about reasons for the Karzai meeting being canceled, reporters traveling with Obama were escorted outside the air field hangar to get a glimpse of the conditions. The wind was blowing strongly, kicking up dust clouds as troops streamed in to hear Obama. An American flag whipped against its pole. At the presidential palace, U.S. armored vehicles were securing entrances. Carpets were ready to be unrolled.

The war in Afghanistan is the nation's longest after Vietnam, launched in the weeks after the Sept. 11, 2001, attacks. This has been the deadliest year to date for U.S. forces in Afghanistan. More than 1,300 have died here since the war began, more than 450 in 2010.

The visit comes a year after Obama announced he was sending an additional 30,000 troops to try to gain control — and then get the United States out — of a worsening conflict. Obama's plan is to start pulling U.S. forces out of Afghanistan in July.