The troops were away when another reporter and I strayed through a fence into a Burmese army camp in a high-security zone south of Rangoon. We had been looking to see ordinary life in tiny Eindayaza village, but found ourselves instead with a rare view of a Burmese military base.

The camp, which is near the village Baptist church, seemed little to look at at first: flimsily-built huts of wood and bamboo that looked as if they would blow down in the next wind were clustered around a wooden building.

This was a bleak spot, but comforts are scarce in Burma. Regular soldiers in the 400,000-strong army are as poor as they are reputedly fierce - the lowest-paid men have to struggle to afford a proper pair of boots.

"Conquer thine enemy," read a flowery title on the largest of a few maps and documents pinned to a wall in the main building. Blue rectangles dotted around a bright yellow line showed the positions of around 2,400 soldiers guarding a 40-mile onshore section of the £0.75 billion Yadana gas pipeline.

The almost-finished mega-project being built by French oil giant Total and California's Unocal will soon be joined by a second pipeline running from Burma into Thailand which Britain's Premier Oil will start laying later this year.

The oil companies have steamed ahead in Burma despite a chorus of protest from human rights groups, which accuse them of supporting a brutal dictatorship. The groups say soldiers guarding the projects have perpetrated massive abuses on local villagers, including torture, rape, murder, displacements of whole villages and forced labour.

Total and Premier flew a group of journalists in recently for a brief view of the pipeline and to defend their projects. The Light Infantry Battalion 409 camp we had blundered into was not on the agenda, and our intelligence officer escort later told other reporters it was off-limits.

The oil companies balk at being held to account over allegations that forced labour is being imposed on local people by soldiers from LIB 409 and other regiments providing pipeline security.

Forced labour is actually "normal" throughout the country. Amnesty International and a host of other human rights groups have condemned Burma for forcing millions of ordinary people to do backbreaking hard labour, stacking and breaking rocks for roads and railways.

Civilians are also made to work as porters for the army, sometimes in life-threatening situations.

Critics say the oil companies' presence in southern Burma has increased and encouraged the use of forced labour here.

Video evidence smuggled out shows villagers doing forced labour on the nearby Ye-Tavoy railway used by troops guarding the pipeline. Soldiers are also responsible for building new roads in this little-developed region.

The soldiers in Light Infantry Battalion 409's army base were clearly supervising some local infrastructural work. Among the documents pinned to the wall was a neatly-drawn wall chart which monitored the "Progress of Rock Collecting". Hundreds of rock piles were numbered and accounted for, although it was not clear where the work was taking place.

LIB 409 turns up in testimony from people who fled this region for refugee camps in Thailand. Mr Maung Dtoo, a LIB 409 deserter, said he had helped burn down a village, conscripted villagers to work as army porters, and watched in horror as superiors raped two Mon women.

Unocal is fighting two pipeline-related lawsuits in Los Angeles. Burma's pro-democracy leader, Ms Aung San Suu Kyi, last week described Premier Oil as "very selfish" for doing business with the military regime.

The lack of access to the normally forbidden region has made it extremely difficult for reporters to write about. Rare visitors must be flown in by the oil companies for what are usually very brief views. The Burmese military does not allow reporters or anyone else free rein around the wider region.

The oil companies say they do not know what the soldiers are doing and that they are helping develop a chronically poor area by paying above-average local wages and handing out aid to villages on the pipeline route.

Mr Ronald Morris, Premier's local general manager, said Premier was spending $1.4 million (£875,000) over three years on community development projects here.

Flying in the face of a call from Ms San Suu Kyi for non-governmental organisations to stay out of Burma, Save the Children (USA) has been hired for $350,000 (£200,000) a year to set up education programmes.

Total is helping 13 designated villages with shrimp farms, poultry and pig projects, schools and health programmes, said the company's new local general manager, Mr Michel Viallard.

A map in the Eindayaza camp detailed each house in the village and the names of heads of household, indicating extraordinary army interest in this tiny ethnic Karen settlement. Karens have been traditionally opposed to Burma's military government, and they are the only ethnic group left which has not signed a ceasefire agreement with Rangoon.

Ten villagers from Eindayaza were summarily executed by troops from another regiment two years ago, in retaliation for an attack by unknown perpetrators on the pipeline, according to human rights group Earthrights International (ERI).

Forced labour and other abuses are still continuing in the region, says Mr Ka Hsaw Wa, a Karen director of ERI, which is filing one of the two California lawsuits against Unocal.

According to a recent Unocal submission to a US Department of Labour inquiry into labour practices in Burma, "the government of Myanmar does not provide or arrange for personnel to work on the pipeline." But a 1996 Total document handed to a US official appears to contradict that.

It describes "payments made to villagers hired by the army." In Burma, people "hired" by the army rarely feel they have a choice in the matter, even if they are paid.

Progress has not been smooth for the Yadana project on the Thai side of the border either. Prominent social activist Sulak Sivaraksa was arrested in a storm of publicity in March for refusing to quit a protest site where the pipeline cuts into pristine Thai forest.

Now he is planning to use his May trial to condemn consortium partner the Petroleum Authority of Thailand and the multinationals for their record on the environment and human rights.

That will irk the oil companies once more. But it will not stop the race to finish the work by next July.