Tensions between the United States and Pakistan continue to heighten. US forces in Afghanistan are suffering casualties from militants operating beyond the tribal areas of Pakistan along its border with Afghanistan.

Following the Marriott Hotel bombing on September 20, Pakistan's president told Parliament that Pakistan would not tolerate the violation of its sovereignty and territorial integrity by any power in the name of combating terrorism.

US President George W.Bush and his Pakistani counterpart, Asif Ali Zardari, met in New York on September 23 to discuss cooperation in combating terrorism. President Bush acknowledged Pakistan's concerns about violations of its sovereignty by US forces, but avoided any commitment to desist.

Behind this exchange between heads of state are reports that in June President Bush secretly approved counter-terrorist raids by US special forces inside Pakistan against al-Qaeda and Taliban terrorist targets, notably Osama bin Laden and his deputy, Ayam al-Zawahiri.

Requests by US theatre commanders for more liberal rules of engagement are likely to have been under consideration for some time. Hitherto, Pakistan has apparently countenanced the CIA's use of remotely piloted Predator missile-firing vehicles in its airspace, but restricted ground personnel activities to the CIA, excluding conventional US military personnel.

Extant US rules of engagement doubtless include the right of US forces to act in self-defence inside Afghanistan, but may also extend to the accepted right of hot pursuit, in specific limited circumstances, of militants fleeing across the border into their havens in the tribal areas.

Confusion surrounds the question whether US and Pakistani forces have exchanged fire in various incidents. Reports of civilian casualties allegedly attributable to operations by US special forces inside Pakistan are accumulating.

Where is this heading in terms of international conflict management?

First, considerations necessarily flow from the dual bases of the presence of US forces in Afghanistan and the practicalities surrounding the mutual self-protection of US and other international forces in Afghanistan, including Australian forces.

The US has a significant NATO presence in Afghanistan, as part of the International Security Assistance Force.

The force is supported by resolutions of the UN Security Council aimed at undertaking reconstruction, nation-building and security stabilisation in cooperation with the Afghan Government.

Additionally, for US forces only, there is a separate bilateral arrangement between the US and Afghanistan, underpinning the belatedly revamped Operation Enduring Freedom.

Afghanistan's security, sovereignty, independence and territorial integrity are deemed particularly crucial areas for US-Afghan cooperation.

These separate bilateral assistance arrangements were settled between President Bush and President Hamid Karzai in 2005.

Pakistan has the clear prerogative, and obligation, to prevent its sovereign territory from being used as an operating base and sanctuary by Taliban and al-Qaeda militants. To this end, Pakistan has deployed substantial numbers of troops in the border region. It may need extra support in these efforts.

That said, it seems Pakistan in the immediate, but critical, future may continue to be unable to prevent raids and acts of terrorism associated with its tribal areas.

Claims might conceivably be made by the US that it has been invited to assist neighbouring Afghanistan: the threat from Pakistan's border region to the security of Afghanistan to ISAF's forces there by invitation of the Afghan Government, is clear and material.

The right of mutual self-defence can obtain between the International Security Assistance Force, US forces in Operation Enduring Freedom and the Afghan Government's own forces. The US could point to many and repeated requests to have Pakistan deal with the terrorist problem in its tribal areas.

Limited cross-border actions in self-defence by US forces would manifestly violate Pakistan's sovereignty.

Pakistan would be fully entitled to an armed response if it chooses. But, in principle, such incursions would seem legally sustainable to protect the International Security Assistance Force, Afghan and US forces engaged in Operation Enduring Freedom.

The more controversial, and inextricably interwoven, dimension to US cross-border incursions is the issue of preventing the border regions from being further consolidated as terrorist havens with potentially global consequences.

For what the US might countenance unilaterally in its global campaign against terrorism (albeit in simultaneously protecting Afghanistan and those international forces assisting Afghanistan's Government) is substantially beyond the International Security Assistance Force mandate and Operation Enduring Freedom within Afghanistan.

The universal international obligation and authority to root out terrorism was reinforced by Security Council resolution 1368 in the aftermath of September 11, 2001, and remains extant.

In the context of the attacks on the US, Resolution 1368 recognises the inherent right of individual or collective self-defence in accordance with the UN charter, and therefore resultantly characterises terrorist attacks of such gravity as ''armed attacks'' under international law.

Arguably, self-defence would extend to operations mounted in anticipation of an imminent armed attack by terrorists, manifested by sound intelligence about the training or planning of transnational terrorist operations.

It would not be required that such intelligence be shared with affected states' intelligence services, in an attempt to achieve local cooperation in dealing with the threat.

Resolution 1368 needs rigorous factual support in its continued application, especially seven years after it was passed. But it certainly stands as a guide to decision-makers, especially in any contingency concerning terrorism associated with the acquisition of weapons of mass destruction.

A spectacular and highly risky option would be for US forces to physically try to apprehend key terrorist operatives inside the tribal havens and remove them to face legal processes. Arm's-length tactics, using remote shooting platforms or conventional munitions otherwise delivered by cruise missiles or aircraft, are the most practicable and adequately compliant self-defensive option.

Reports of civilian casualties emerging after some raids attest to the risks endemic in these activities. Both Afghanistan and Pakistan are evidently under immense pressure. It is in the interests of the international community that this acute situation be immediately brought under positive control, ideally through the Security Council, as it poses a major threat to international peace and security.

Dr Michael Smith is a Canberra barrister and international lawyer