By Nyime Amorh

YORLA OIL FIELDS, Nigeria (Reuters) - Through the thick white fog stinging with the smell of raw gas, boys clutching handkerchiefs to their noses patrol making sure no one starts an engine, takes a flash photograph or does anything that could light a spark and ignite the whole area.

The continuous hiss of rushing gas, so loud that people within an arm's reach have to shout to each other, can be heard 500 metres (yards) away from the site of the oil wellhead that burst on Sunday on an Ogoniland farm in Nigeria's southeast.

One hundred metres from the site people started choking, their noses running from the acrid fumes of gas.

Shrubs surrounding the "Christmas tree" shaped wellhead itself were drowning in a growing pool of crude oil as wide as a swimming pool. Oil gushed from the base of the Christmas tree with such force that the pool looked as if it was boiling. It threw off a brownish vapour that turned grey as it rose into the air.

"We are sitting on a keg of gunpowder," Chief Dumle Nwiabu told Reuters as he surveyed the scene. The local community said a downpour on Thursday morning had helped dissipate the thick cloud of gas and bought the three villages neighbouring the farm more time.

But late on Thursday, Nwiabu said he had seen no sign of the U.S. oil well specialists who were to shut down the wellhead, nor the medical team his people had requested from Royal Dutch/ Shell, the one-time operators of the well.

Children were having coughing fits, residents were complaining of chest pains and having difficulty breathing, Nwiabu said.

But his biggest fear was that a spark could turn the pool of oil and cloud of gas into an inferno.

He had dispatched 160 youths to keep watch over a three km (2 mile) radius around the spill and make sure no one did anything that could start a fire.

The spill has renewed hostilities between the Ogoni people and Shell which was forced out of the area in 1993 after the tribe argued that oil exploitation had devastated their environment and left them in poverty.

The dispute attracted the world's attention when the leader of the Movement for the Survival of the Ogoni People (MOSOP), writer Ken Saro Wiwa, was hanged with eight other Ogoni activists on murder charges in November 1995 by Nigeria's then military junta.

MOSOP complained on Thursday that Shell was not moving fast enough to save local villagers from disaster.

"This is a serious and ongoing oil blow-out which deserves sensitive and comprehensive action by Shell to prevent a disastrous fire," MOSOP president Ledum Mitee told reporters in nearby Port Harcourt.

A spokesman for Shell's Nigerian Unit told Reuters that because of the specific type of spill, Shell had had to wait almost five days for specialists from Boots & Coots International Well Control to arrive from Houston to stop the gushing oil.

"When you have an ordinary spill, we can just go in and clean it up. But in this case the wellhead has been tampered with and there is all this built-up pressure," the spokesman said by telephone from Lagos. "It's like shaking a bottle of Coke and prying the lid off."

Shell's Port Harcourt spokesman Donald Boham told reporters at a briefing on Wednesday that the spill at the Yorla field was proof that all the unsecured wells in the area it had abandoned hastily nine years ago because of unrest could blow without warning.

Only two of the 16 wells in the Yorla field were "properly abandoned" in 1993 after the Ogoni people forced Shell out of the region.

"Fourteen are still potential time bombs," Boham said. "We withdrew from Ogoni without being allowed to carry out proper evacuation procedures," he said.