"Shell has received reports of a clash between soldiers of the Joint Task Force and some militants around the Brass Creek last (Sunday) night, resulting in a number of injuries and casualties," the Anglo-Dutch oil giant Shell said in the statement, sent to AFP.
"There are indications that an SPDC (Shell Petroleum Development
Corporation) staff (member) who had been taken hostage in the Letugbene area of Bayelsa State may have been affected in the incident," it said without giving details.
His whereabouts are still unknown, it added. The statement did not explain what led to the clash in southern Bayelsa State.
Meanwhile, Nigerian security forces have stepped up efforts to free five foreign oil workers still held by militants in the swamps and creeks of the oil rich Niger Delta.
"We are not relenting in our efforts. Everything is being done to secure the release of the remaining hostages," army spokesman Major Musa Sagir said.
He said the Joint Task Force which raided the region and arrested some 100 suspected militants over the spate of kidnappings in recent weeks had released around 50 who were deemed not to be involved.
The security clampdown at the weekend followed a directive from President Olusegun Obasanjo to flush out suspected kidnappers terrorising oil firms, staff and facilities in the oil rich but restive region.
More than 40 foreign oil workers have been kidnapped since January, with around 15 abducted in the past two weeks.
Ten of them have been released while five -- an American, a Briton, a German, an Irishman and a Lebanese construction worker -- are still in captivity.
Nigeria, Africa's biggest oil producer, is the world's sixth biggest crude exporter with 2.6 million barrels per day, but 20 percent of that figure is lost to unrest in the Niger Delta.