WASHINGTON, March 3 (Reuters) - U.S. forces will need "at least a few more months" to clear al Qaeda militants from the northern city of Mosul, the Sunni Islamist group's last urban stronghold in Iraq, a senior U.S. Army officer said on Monday.

But success against al Qaeda in Mosul is posing a new challenge for U.S. forces, as military operations drive militants from the city to remote areas beyond reach of U.S. and Iraqi intelligence and surveillance.

"The real challenge now is to find where they are with any sort of center of gravity," said Army Brig. Gen. Tony Thomas, second in command of multinational forces in northern Iraq.

Coalition and Iraqi security forces have established joint-security sites and combat posts in the heart of Mosul as "the first critical step" toward lasting security.

"Kinetic operations will last for at least a few more months," Thomas told Pentagon reporters in a videolink from a military base near Tikrit.

"We're slowly but surely eliminating their (al Qaeda's) toe-hold in the city," he added. "We have the upper hand and the initiative in the city right now."

Mosul, an ethnically and religiously mixed city 240 miles (390 km) north of Baghdad, became a destination for al Qaeda militants driven from Iraq's western Anbar Province by Sunni tribesmen who switched their alliance to the United States last year.

Tens of thousands of U.S. and Iraqi forces are now carrying out offensives against al Qaeda in central and northern Iraq, and Iraqi Prime Minister Nuri al-Maliki has promised there would be a "decisive push" against its fighters in Mosul.

Thomas estimated that 10 percent to 15 percent of al Qaeda's forces consist of foreign fighters, including Saudis. But he declined to estimate the number of militants.

"The good news story is we're forcing them out of the city, we're disrupting their focus and their effort there, and we're continuing to apply pressure on them as they fall back," he said.

But the outlying villages and desert hide-outs al Qaeda miltants are fleeing to are areas where U.S.-led forces have no intelligence, reconnaissance or surveillance systems set up to track their movement.

"We have to adapt our assets a little bit to that challenge," he said.

(Reporting by David Morgan; editing by Stuart Grudgings)

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