The announcement on Saturday came as the Americans have been pressing their hunt for the al-Qaida leader and his top deputy, Ayman al-Zawahri, who are believed hiding out in the border area between Afghanistan and Pakistan. 

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

The operation, however, was "about more than one person," he said. Hilferty said American forces had "confidence" they would eventually catch bin Ladin, but not necessarily during the new operation. 

'Mountain storm'

Hilferty said US forces were involved at the moment in what he described as a "small scale air assault" in southern Afghanistan, but would not say where it was taking place or give details about the target. 

The operation, dubbed "Mountain Storm" officially began on Sunday and was open-ended, Hilferty said. He said the entire 13,500-strong US-led coalition was involved, though he gave no specifics about their role. 

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

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

Pakistan operation

Ayman Al-Zawahri is Usama
bin Ladin's  top deputy

About 70,000 Pakistani troops have moved into semi autonomous tribal regions in an effort to cut down hiding space for al-Qaida and Taliban fugitives, believed to have taken refuge there. 

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

Pakistan, a key ally in the US 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 organising operations from large Pakistani border towns like Quetta and Peshawar. 

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

'Spring offensive'

"We believe this will help bring the heads of the terrorist organisations to justice, by continuing placing pressure on them."
Lt. Col. Bryan Hilferty, 
US military spokesman

Hilferty played down suggestions by defence 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 the media have been calling for, not us." 

The new US military offensive is also supposed to safeguard landmark Afghan elections set for June, when US-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 in advance of the vote.