More than 30 foreign workers are being held by militant groups in the restive delta. Hostages are often released after negotiations with kidnappers, culminating in the payment of ransom or other economic benefits.
At least one of the gunmen who seized the employees of the Chinese National Petroleum Company was killed in a shootout with security forces. An unspecified number of militants were also reportedly seriously injured.
Joshua Benamiasia, leader of a group of Nigerian negotiators who had secured the release of hostages in the past, says the deadly confrontation could complicate the fate of the Chinese hostages and make negotiations for their release more difficult.
"It would have been better if they [security forces] had killed all of them [militants] and not let them escape with the hostages," he said. "But now that they have taken hostages and they have casualties on their side, it won't be easy on the hostages at all. But at the same time,we'll see what we can do."
The Movement for the Emancipation of the Niger Delta, the region's leading separatist group, is demanding the release of two jailed ethnic Ijaw leaders, in addition to a greater share of Nigeria's oil revenues.
Meanwhile, the Philippines has asked Nigeria not to contemplate attempts to rescue 24 Philippines seized from a ship off Nigeria last week.
Manila is worried such an intervention could put the hostages in harm's way and complicate current negotiations.