The declaration comes 10 months after thousands of U.S.-led NATO troops stormed the cluster of farming hamlets to oust the insurgents and cut off their income from the drug trade.
The campaign took longer than NATO officials had hoped, and underscored the complexity of trying to wrest control of an area where Taliban influence remained strong.
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Maj. Gen. Richard Mills told reporters at the Pentagon on Tuesday the enemy has been pushed to the outskirts of the area, where insurgents come in from the desert to take "the odd shot at us." In the more populated areas, Mills said Afghan police are mostly providing the security on their own.
Mills said tough fighting continues in other parts of Helmand province, including the Sangin district where Marines have taken over a bloody battle from British forces.
He vowed an "aggressive winter campaign" to blunt the potential of a revival by the Taliban next spring.
"We will move into areas we thought were unreachable by coalition forces. We will give them no rest," Mills told Pentagon reporters via video link from Afghanistan.
The assessment comes as President Obama is evaluating his war strategy since committing an additional 30,000 U.S. troops to the fight last year.
The report, which could be made public next week, was expected to cite modest gains against the Taliban that could let the U.S. start to bring some troops home. Mr. Obama has already said that a reduction must begin next July.
But the assessment also was expected to acknowledge that the military has not completely secured even small districts in southern Afghanistan - a necessary milestone before U.S. troops could begin pulling out in significant numbers.
Marjah has become an example of the difficulty facing the military campaign. More than 7,000 ground troops launched a massive nighttime invasion last February that NATO officials said would pave the way for the Afghan government to rush in aid and restore public services.
But the civilian component to rebuilding Marjah was painfully slow, and U.S. troops struggled against roadside bombs and sniper attacks from an enemy that could blend in with the local population.
Mills declined to say how soon it might be before U.S. troops could be withdrawn in significant numbers from Helmand province, noting that heavy fighting continues in some areas outside Marjah.