The decision to deploy warships around the platforms came as talks on the crisis between Transocean Inc and a Nigerian trade union broke down without agreement, a navy spokesman said.

Captain Sinefi Hungiapuko said the navy would not try to storm the platforms, and would seek a peaceful resolution to the crisis. But the military intervention will raise tensions in the 11-day stand-off.

"The navy is directly involved now," he said. "We are trying to make sure things are settled peacefully. The navy is in the area. Our men will move to take control of the rigs ... immediately."

Asked what the navy would do if it encountered resistance, Hungiapuko asked: "What action would you expect military men to take?"

But Transocean spokesman Guy Cantwell said naval officials told the company they were not moving on the rigs.

"We have not seen any ships at our rigs from the navy and the navy has told us that they are not sending their ships to our rigs," Cantwell said from company headquarters in Houston, Texas.

Nigerian workers on the four oil platforms lying in the Gulf of Guinea in deep water south of the Niger Delta launched a protest April 19 after five union colleagues were sacked for alleged theft.

They have blocked the helicopter landing decks and restricted access to the rigs from the sea. At the start of the crisis at least 97 foreign workers, mainly British, American and European oilmen, were on board.

Between 30 and 40 foreign and Nigerian workers have been permitted to leave during the crisis, according to the unions and Transocean. It is not clear how many expatriates remain.

Nigeria's National Union of Petroleum and Natural Gas workers said the conflict over the sackings was over, but that workers were still refusing an order to evacuate the rigs.

Union president Peter Akpatason said the workers were prepared to leave the platforms under a standard crew changeover, but fear they will be sacked if they leave for shore without being replaced by another crew.

"The meeting was blocked because management was not ready to change its rigid position. They are not ready to shift ground," he said.

E-mail messages from the trapped workers have warned that the strikers have threatened to blow up the rigs and kill everyone on board if Transocean attempts to take them back by force.

But industry and diplomatic sources in Nigeria have played down such threats and foreign missions here do not regard the trapped oilmen as hostages.

"Throughout this strike our reports from the rigs ... have consistently said the situation remains relatively calm," Cantwell said, adding that people are free to move around the rigs, and the striking workers who control access are not blocking shipments of food and water.

The deployment of the navy will raise tensions, however.

Akapatason said that his union would advise against such an operation.

"If they don't comply with this we will be forced to take appropriate action," he said, warning of an extension of the strike.

Transocean has won a court injunction ordering the strikers off the platforms and has retained four bailiffs, each accompanied by three armed police officers, to enforce the order.

The workers have comandeered four platforms, the Sedco 709, the MG Hulme, the Trident 8 and the Trident 6, in three areas along the southern Nigerian coast, two of them in deep water and one on the continental shelf.

Three of the platforms -- one in the deep water Bonga field and two on the continental shelf in the EA field just off the western Delta in the Gulf of Guinea -- are leased by the Anglo-Dutch oil group Shell.

One deepwater well off the eastern Delta in the Bight of Bonny is leased by France's TotalFinaElf.

A Shell spokesman said some of its expatriate employees were among the trapped workers, but that it was not involved in the talks except in an advisory role.

Nigerian oil facilities are often taken over by strikers or by local militants keen to extract lucrative concessions from the wealthy foreign companies that produce the country's sole profitable export.

It is rare for expatriate staff to be hurt in these incidents.

Last month, violent unrest in the western Niger Delta forced three oil multinationals to suspend their operations, stopping more than 40 percent of Nigeria's daily exports of crude.

Nigeria is the world's sixth largest oil exporter, with an OPEC quota of more than two million barrels per day.

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