"I have no intention of sending US boots on the ground in these regions."
He insisted the lawless tribal belt straddling the Afghanistan-Pakistan border "remains the epicenter of Al-Qaeda," but acknowledged a Yemen-based affiliate of Osama bin Laden's network has become "a more serious problem."
The impoverished country's long-standing scourge of extremism was thrown into the spotlight after the Al-Qaeda branch claimed responsibility for a narrowly-averted Christmas Day bombing aboard a US-bound airliner.
Recent strikes on Al-Qaeda positions in Yemen, including cruise missile attacks, were reportedly led by the United States, which has vowed to boost its economic and military aid to Sanaa. London and Washington have already announced plans to fund a counter-extremism police in the country.
Mullen, the Chairman of the Joint Chiefs of Staff, acknowledged in a CNN interview that the United States was providing "some support" to Yemen's efforts to strike Al-Qaeda militants, but insisted Sanaa led the operations.
Yemen has been hostile to any US military intervention, but analysts fear bin Laden's ancestral homeland cannot tackle the militants on its own.
Striking a conciliatory tone, Obama said the message his administration sends to Muslim communities around the world was "extraordinarily important."
"We can't return to sort of a garrison-state notion that we're just going to hunker down and this is only an issue of firepower and boots on the ground," he added.
A thinly stretched US military has deployed large troop contingents in Iraq and Afghanistan.
The number of US troops in Afghanistan -- where Obama has vowed to focus his war against Al-Qaeda militants who have also sought refuge in neighboring Pakistan -- is set to triple under his watch from 2008 levels, reaching some 100,000 later this year.
Washington has urged Yemen to crack down on Al-Qaeda but Sanaa already faces a litany of challenges, including a water shortage, dwindling oil reserves, a Shiite rebellion in the north and a movement for autonomy in the south.
Somalia is also the focus of US counterterrorism efforts, where an embattled transitional government faces relentless attacks from extremist Shebab militants and their Hezb al-Islam allies.
The central government asserts little control over the country located along key shipping routes to oil fields in the Middle East, with pirates now swarming the Gulf of Aden and the Indian Ocean.
US officials have said they are seeking to boost military and intelligence cooperation with Yemen.
General David Petraeus, the head of US Central Command, which oversees a region encompassing the Middle East, the Gulf, the Horn of Africa and Central Asia, welcomed Yemen's desire to tackle extremists on its own.
"We would always want a host nation to deal with a problem itself. We want to help. We're providing assistance," he told CNN after returning from a trip to Yemen during which he held talks with President Ali Abdullah Saleh.
Petraeus said Washington planned to more than double its economic aid to Yemen this year to 150 million dollars or more, up from 70 million last year. But US officials have insisted the total aid amount has not yet been determined.
Though the figure pales in comparison to the billions of dollars Washington has poured into Afghanistan, the general stressed other allies were providing aid, including Saudi Arabia, which has reportedly allocated two billion dollars, and the United Arab Emirates, which pledged 650 million dollars to Sanaa.