ISLAMABAD The furore has intensified over Washington's decision to pursue militant targets inside Pakistan, with opposition lawmakers threatening the country could pull out of the war on terror if the US refuses to respect its borders.

About 100 protesters burned American flags Friday after the latest missile attack left at least 12 people dead in North Waziristan.

Residents said they heard the sound of propeller-driven US Predator drones circling overhead before the explosions.

US President George W. Bush secretly approved more aggressive cross-border operations in July, current and former American officials have told The Associated Press.

Since August 13, there have been at least seven reported missile strikes as well as a raid by helicopter-borne US commandos that Pakistani officials claim killed 15 civilians in tribally governed territory.

The Pakistani government and military have issued stiff protests to Washington over the recent rash of cross-border strikes, although the criticism appeared to be mostly rhetoric aimed at soothing domestic anger, given that Pakistan has few options for stronger action.

The media has criticised the government for not reacting more strongly, even suggesting the public criticism is just lip service and that a secret deal has been reached with Pakistan's leadership allowing cross-border incursions.

The Chief of Army staff General Ashfaq Parvez Kayani has denied that and vowed to protect the country's sovereignty 'at all costs.'

Leaders, including new President Asif Ali Zardari, have reiterated their commitment to fighting violent extremism and have aired no threats to withdraw their cooperation.

However, they are sensitive to public opinion in Pakistan, which is hostile to US policy in the region.

Agitation on the issue by former prime minister Nawaz Sharif could make it hard for Islamabad to maintain the close alliance with Washington forged by Zardari's predecessor, Pervez Musharraf.

'We need at this time to make it clear to foreign countries that Pakistan will not tolerate such actions,' said Ahsan Iqbal, a lawmaker in Sharif's Pakistan Muslim League-N party.

'If it continues, then Pakistan can consider pulling out completely from this war on terror.'

Iqbal and another party leader called for an urgent parliament session to debate how Pakistan can respond.

'The parliament must be convened on a one-point agenda, because the nation is under a threat of war,' said lawmaker Chaudhry Nisar Ali Khan. 'Irrespective of where the threat is, every inch of this country is sovereign. Every inch of this country is sacred.'

Defence Minister Chaudhry Ahmad Mukhtar said Pakistan's armed forces were 'ready to meet any such eventuality if this is repeated' and evoked Pakistan's war against India in 1965.

Despite the strong language, parliament has few options beyond issuing a condemnation of cross-border raids and reiterating the country's sovereignty.