A manager from energy giant, Shell, who asked not to be identified, told AFP that guerrillas had attacked a pipe-laying barge off the Forcados oil terminal at around 5:00 am (O400 GMT) and had taken the men hostage.
"The attackers came in five boats. We don't know the extent of the damage, but two soldiers were reported injured," he said.
Shell's operations in the delta are protected by troops from a regional military task force.
The kidnapped expatriates were working for a Shell sub-contractor, the US engineering firm Willbros, on a pipe- laying barge operating in the Forcados river, 50km (30miles) west of the oil city of Warri.
Forcados is one of Shell's two main export terminals in Nigeria and if the attack on the loading platform is confirmed then exports totalling hundreds of thousands of barrels per day will be hit.
"The attackers came in five boats. We don't know the extent of the damage, but two soldiers were reported injured"
a manager from Shell
The militants claimed that they had killed five soldiers in the attack, but this could not be independently confirmed and military spokesmen said they were not yet ready to give details of what happened.
"There was an incident like that, we're monitoring the situation. We don't yet have details of what happened," said Obiara Medani, the navy's Captain.
The rebels identified the captives as three Americans, a Briton, two Egyptians, a Filipino and two Thais and warned that they planned to step up their war on the oil industry in retaliation for government air strikes.
"In response to attacks on communities in the Niger Delta by the Nigerian army, units of the Movement for the Emancipation of the Niger Delta moved into Delta State yesterday to commence an attack on installations," they said, according to Reuters.
The Movement for the Emancipation of the Niger Delta said the employees of US oil services company, Willbros WG.N, were taken in an attack on a barge known as "Barge 318".
"These individuals and facilities were well guarded by a large number of soldiers who resisted for an embarrassingly short period before escaping to ensure their personal safeties," the group said in an email, naming all nine hostages.
In a statement emailed from an address used by the militia, the group also claimed to have severely damaged the export terminal, a major pipeline manifold and a gas pipeline supplying energy to Lagos.
Saturday's attack followed a week in which the Nigerian military carried out two air strikes against barges used by the militants to smuggle stolen crude.
Nigeria is a major oil producer,
but its people live in poverty
Last month, the same gang had kidnapped four foreign oil workers and held them for 19 days, demanding that two ethnic Ijaw leaders be released from jail and that Shell pay $1.5billion (1.2billion euros) to the tribe.
The hostages were eventually released unharmed, but the group and its allies also attacked a number of oil facilities, killing at least 22 soldiers and police in two assaults, and blew up a major pipeline.
Shell's production was already down by 106,000 barrels per day when the latest violence broke out and the company has closed four of its oil flow stations in the western delta because of security fears.
The Niger Delta- a 70,000 square km (27,000 square mile) swathe of swampland and mangrove forest - is home to Africa's biggest oil industry and to the 14-million-strong Ijaw tribe, many of whom dream of independence.
Although Nigeria is a major oil exporter, producing around 2.6 million barrels per day, most of its 130 million people live in grinding poverty and there is much resentment of government and the oil firms in the delta.
Several illegal militias operate in the creeks and jungles of the region and in recent weeks have stepped up attacks on both government forces and the oil industry, in particular the Anglo-Dutch energy giant, Shell.