Nigerian oil workers' unions have threatened to withdraw their members from the troubled Niger Delta as tension built up there following the abduction of four Shell workers and a spate of attacks on oil facilities, a senior union official told Xinhua on Friday.

"They (kidnappers) must release them (Shell workers), if they don't release them and kept on harassing other people, we will withdraw oil workers from the field for their safety," said Elijah Okougbo, general secretary of the blue-collar National Union of Petroleum and Natural Gas (NUPENG).

Okougbo said the NUPENG and the white-collar worker union, the Petroleum and Natural Gas Senior Staff Association of Nigeria ( PENGASSAN), have issued a joint statement "over the restiveness and consequent harassment and violence" in the Niger Delta, where the majority of Nigeria's oil is produced.

News reports have quoted the kidnappers as saying that 61-year- old Patrick Landry, an American, on his 10th day in captivity, was gravely ill and could die, and the other three would be killed if he died.

Landry and three other hostages, a Briton, a Bulgarian and a Honduran, were abducted on January 11 when armed men, riding in three boats, stormed the Shell-operated EA oil field offshore Nigeria's southern coast.

In a statement issued on Thursday, Shell's subsidiary in Nigeria, Shell Petroleum Development Co. (SPDC), said it was cooperating with the Nigerian authorities to secure the safe release of the four hostages.

"We remain deeply concerned about the hostage situation that resulted from the abduction of four Shell contractors from SPDC's EA field operations," said the company.

"We recognize the immense strains placed upon the hostages, their families and contractor companies under the present circumstances... The health and safety of hostages remains our top priority," it added.

Nigeria is the biggest oil producer in Africa with a daily output of 2.5 million barrels, while Shell accounts for half of the country's oil production.

Source: Xinhua