The offensive comes as Americans step up their hunt for the al-Qaida leader and his top deputy, Ayman al-Zawahri, who are believed to be hiding out in the border area between Afghanistan and Pakistan.

"We believe this will help bring the heads of the terrorist organizations to justice, by continuing placing pressure on them," said Lt. Col. Bryan Hilferty, a U.S. military spokesman.

The operation, however, was "about more than one person," he said. Hilferty said American forces were confident they will eventually catch the al-Qaida leadership as well as Taliban chief Mullah Mohammed Omar, but not necessarily during the new operation.

Hilferty also said U.S. forces are involved in what he described as a "small scale air assault" in southern Afghanistan, but would not give details about the location or the target.

The overall operation, dubbed "Mountain Storm," officially began Sunday and was open-ended, Hilferty said. He said the entire 13,500-strong U.S.-led coalition was involved.

While bin Laden's whereabouts have been the subject of intense speculation, there has been no known hard evidence of his location - or even that he is alive - since shortly after the Sept. 11, 2001, attacks on the United States.

Hilferty said the operation was in effect a continuation of tactics already being used, such as intensive patrolling, village searches and impromptu checkpoints.

He declined to give specifics, but an Associated Press reporter at the military's main southern base at Kandahar noted what base personnel said was heavier than usual air traffic, with C-130 cargo planes and Chinook helicopters landing through the night.

The base also served a lobster and steak dinner on the eve of the new operation. The army traditionally serves special meals to kick off large offensives.

Lt. Gen. David Barno, the top American commander in Afghanistan, has said his soldiers are engaged in a "hammer-and-anvil" strategy along with Pakistani forces on the other side of the border.

Some 70,000 Pakistani troops have moved into semiautonomous tribal regions to take away maneuver room for al-Qaida and Taliban fugitives believed to have taken refuge there.

A Feb. 24 operation in Wana, the main town in Pakistan's South Waziristan region, netted 24 suspects, but none were believed to be important al-Qaida operatives.

Pakistan, a key ally in the U.S. war on terrorism, has arrested more than 500 al-Qaida suspects. But Afghans also say they have not done enough to seal the border, and complain that Taliban commanders have been organizing operations from large Pakistani border towns like Quetta and Peshawar.

On Saturday, tribal elders in South Waziristan imposed a 24-hour deadline on a tribe accused of sheltering terrorists to hand over the fugitives or expect an armed force of 600 men to search the area forcibly.

A Pakistani military spokesman, Gen. Shaukat Sultan, would not comment on the new U.S. operation or say whether Pakistani troops were involved in fresh deployments on their side of the border.

Hilferty played down suggestions by defense officials in Washington that the military was embarking on a "spring offensive."

"If it continues past March 21, I assume it will be a spring operation," Hilferty said. "But spring offensive is what they media have been calling for, not us."

The military offensive also is supposed to safeguard landmark Afghan elections slated for June, when U.S.-backed President Hamid Karzai is expected to secure a new term.

More than 140 people have died in violence already this year, underlining security fears ahead of the vote.

Much of the south and east of the country remains off-limits to international aid groups, and local officials complain their forces are unable to deal with the Taliban threat without more help from the Americans and the central government.

Hilferty said the previous two-month U.S. operation, called Blizzard and including 143 raids and searches, had resulted in the death of 22 "enemy combatants." No U.S. soldiers were killed in combat during the period, he said, though a number died in accidents.

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