The US programme of drone strikes targeting Islamist militants in Pakistan, Yemen and other countries violates international law and should be halted, a legal expert warned Thursday.

Mary Ellen O'Connell, a law professor at the University of Notre Dame, told a debate at a leading London think-tank that the pursuit of Al-Qaeda and Taliban extremists should be a law enforcement issue, not a military one.

"The strongest conclusion is that there is no legal right to resort to drone strikes in Pakistan, Yemen, Somalia and elsewhere where the US is not involved in armed conflict," she told the respected Chatham House centre.

She was particularly critical of strikes by the US Central Intelligence Agency in the northwestern tribal zone of Pakistan which borders Afghanistan, a haven for militants who use it as a base to attack NATO and Pakistani forces.

"The use of drones is causing really serious anger in Pakistan. I really seriously question the necessity for what we are doing," she said.

O'Connell said they could not be justified because there was no open consent from Pakistan, and the strikes could not be taken as an act of war because they did not happen on Afghan soil, where US troops operate.

But Michael Schmitt, an international law professor at Britain's Durham University who spent 20 years in the US air force, told the debate that the strikes were "completely within the law of self defence".

He argued that the drone strikes were a valid measure against a new "transnational" form of combatant, and that they could also be justified if the country where they are based either refused or was unable to act against militants.

The United States has dramatically increased the frequency of drone strikes in Pakistan in 2010, particularly in recent months in response to intelligence claims of plots to launch commando attacks on European cities.

US officials say drone strikes are highly effective in the war against Al-Qaeda and its Islamist allies, but their legality remains shadowy and Washington has never publicly acknowledged the existence of the programme.

Pakistan has condemned the strikes as a breach of national sovereignty.