Nine foreign oil workers have been seized by armed militants from a barge in Nigeria's Niger Delta.
Shell operates a number of installations in the Niger Delta
The group, including three Americans, two Thais, two Egyptians, a Briton and a Filipino, were on a pipelaying barge.
Shell's Forcados export terminal was also set on fire, and oil loading there has been suspended.
The attacks come a day after a militant commander told the BBC his group was declaring "total war" on all foreign oil interests in the Delta.
The Movement for the Emancipation of the Niger Delta gave oil companies and their employees until midnight on Friday night to leave the region.
In an e-mail quoted by Reuters news agency after Saturday's attack, the group threatened action "on a grander scale".
"We decided in response to pleas from our kin in these communities, to carry out strikes against oil and gas facilities in Delta state," the group said.
Nigerian President Olusegun Obasanjo meanwhile has been meeting senior ministers and security chiefs to discuss the situation.
The attack came at 0500 (0400GMT) on Saturday, on a barge off the Forcados oil facility.
Two soldiers from the military force which protects oil workers in the delta were reportedly injured.
"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," said the militants in their e-mail statement.
The men were working for Willbros, a US engineering firm which is a Shell subcontractor, in the Forcados River, 50km (30 miles) west of Warri.
The Briton has been named by the UK Foreign Office as John Hudspith, originally from the north of England and now living in the South East.
Forcados, located 5km (three miles) offshore and pumping 380,000 barrels per day, is one of the two main export facilities operated by Shell in Nigeria.
Loading at the terminal was suspended following the attack and the scale of damage is being assessed.
On Wednesday the Nigerian military used a helicopter to hit barges it said were being used by militants to smuggle stolen oil.
The rebels recently blew up two oil pipelines, held four foreign oil workers hostage and sabotaged two major oilfields.
The group wants greater control of the oil wealth produced on their land.
Nigeria is Africa's leading oil exporter and the fifth-biggest source of US oil imports, but despite its oil wealth, many Nigerians live in abject poverty.