Democratic and Republican lawmakers on the Senate Armed Services Committee pressed Defence Department officials to clarify whether international law would allow US forces in Afghanistan to strike at ungoverned areas of Pakistan where US intelligence says al-Qaeda is rebuilding.

Republican Sen Jeff Sessions of Alabama suggested the United States could act under the same legal argument used to justify the 2001 US-led invasion that toppled Taliban rule in Afghanistan after the Sept 11 attacks.

"You either are supporting these people or you're not assuming control over your country. Either way, we cannot wait," Sessions told Defence Undersecretary for Policy Eric Edelman at a hearing.

Sessions was joined by other lawmakers including Democratic Sen. Evan Bayh of Indiana: "What (Pakistani leaders) need to contemplate is which is harder for them -- acting to do something about this, or us acting to do something about this."

A US-Pakistani agreement allows US forces to attack pro-Taliban militants across the border in Pakistan if the extremists have just carried out an attack in Afghanistan or pose an imminent threat.

But Edelman told lawmakers that al-Qaeda locations are in regions so remote that no power since Alexander the Great has been able to exercise authority there.

"I don't think that the situation we face right now ... (has) risen to the level that you just described," he replied to Sessions.

The spotlight on Pakistan has intensified in recent weeks, following US reports that al-Qaeda leader Osama bin Laden and his second-in-command Ayman al-Zawahri are establishing camps in the South Asian country.

President George W. Bush is also under pressure from the Democratic-controlled Congress, whose leaders accuse the US administration of losing sight of al-Qaeda while prosecuting the war in Iraq.

This week Vice President Dick Cheney urged Musharraf, a US ally, to take action against al-Qaeda and stop the flow of Taliban insurgents heading into Afghanistan for a spring offensive.

US officials say attacks by pro-Taliban militants in Afghanistan along the Pakistan border have risen sharply since September, when Musharraf struck a deal with North Waziristan militants to halt attacks on the army and stop bordering crossings.

Pakistan has lost more than 700 troops during three years of fighting in the region.

Sen Carl Levin of Michigan, Democratic chairman of the Senate committee, criticised Musharraf for not acknowledging outright that the agreement had failed.

Levin said the panel would press the Defence and State departments for clarity on whether US troops under Nato could act against al-Qaeda camps in Pakistan based on information that attacks inside Afghanistan have been planned at the sites.

"It's a critically important point, and I think we've got to insist, on this issue, that we be given a clear answer," Levin said.

Retired US Marine Gen James Jones, former top Nato operational commander in Afghanistan, said Nato forces might not have member authorisation to take action in Pakistan.

But he told Levin that forces under the US command called Operation Enduring Freedom have a legal right to strike across the border.

"That mission, everybody agrees, could be done," Jones said.