The news report said the Humvee was 'leading' a convoy of military trucks filled with Philippine marines in Indanan, Sulu province. The remote town was the scene of fighting last week between Muslim rebels and the Philippine military, in which 26 soldiers were killed.

'I want to find out what exactly is the extent of involvement of United States armed forces personnel there,' the new Philippines defence secretary, Gilbert Teodoro, said yesterday as the issue revived a debate about the extent of American intervention in domestic affairs.

'I will find out if that convoy was an offensive operation or an administrative run.'

Under the US-Philippine Visiting Forces Agreement, US troops may provide only training, information and humanitarian aid.

Left-wing lawmaker Crispin Beltran warned that should an American soldier get wounded or killed, the US might retaliate by pouring in more troops, exacerbating the situation.

Philippine marines are hunting Abu Sayyaf, a homegrown Muslim extremist group with links to al- Qaeda and Jemaah Islamiah.

But Mr Teodoro, who belongs to President Gloria Macapagal-Arroyo's ruling coalition, seemed inclined to give the foreign troops the benefit of the doubt.

'I do believe that whenever the Americans go from place to place, they are in military movement for their own security,' he said.

A US embassy spokesman said US soldiers travelled with military convoys but were not involved in combat. But he said they could 'fire back if fired upon'.

'Our role is to advise and assist the Philippine military,' the spokesman said. 'This is the main focus of our anti-terror campaign.'

Early this week, Major-General Ruben Rafael, an army commander in Jolo, said his troops were being 'backed' by about 100 US troops who were providing him with 'tactical intelligence'.

But Mr Beltran said he did not believe them. 'Both the Philippines' top brass and the US military leadership in the country are lying about the US troops not being involved in this offensive,' he said. 'It is evident that the very presence of the US troops in the areas where the search and attack operations are being conducted implies involvement.'

Renato Reyes, secretary-general of the left-wing labour federation Bayan, was suspicious of the offensive's intent: 'Is the Sulu offensive intended to give American troops 'on-the-job training' in so-called counter-terror operations?'

The offensive comes as the US Congress deliberates on how much funding to give the Philippine military next year. Recently, it was reported that the Bush administration had slashed its proposed defence funding to US$11.1 million next year from US$17.6 million this year and US$30 million last year.

Intensive lobbying by the Arroyo administration restored the amount last week to US$30 million, Brian Yamsuan, a spokesman for the department of interior and local governments, said.

In May, senior US State Department official Christopher Hill stressed the importance of a continued US presence in Mindanao. US soldiers have been credited with helping find and kill leaders of the Abu Sayyaf, notably its chief, Khaddafy Janjalani, and its notorious spokesman, Abu Sabaya.

US troops numbering from several hundred to a few thousand have been stationed in the south for five years despite a constitutional ban on foreign bases without a signed treaty. But there has been no significant public outcry against their presence.