Two different armed groups have lifted sieges of two oilfield stations in Nigeria, releasing at least 20 workers, industry spokesmen said on Thursday.
About 18 workers at Agip's 40,000 barrel-per-day (bpd) Tebidaba oilfield were released on Tuesday after five days in captivity, while five people at Shell's 14,000 bpd Nun River facility were freed on the same day after a 12-day siege.
A Shell spokesman said the company had begun to ramp up production from Nun River, but industry sources said Agip's Tebidaba facility was still not pumping.
"Nun River reopened on Dec. 26 and production is ramping up," a Shell spokesman said.
Another four oil workers -- three Italians and one Lebanese -- are still being held hostage by a different armed group after an attack on Agip's Brass River export terminal on Dec 7.
Company spokesmen said it was unclear what led to the lifting of the two oilfield sieges, but that talks with the attackers were led by the state government and involved elders from the communities.
The attackers had demanded money, jobs and infrastructure for their communities.
Kidnappings and attacks on oil facilities have become an almost weekly occurrence in the world's eighth largest exporter, and Western oil companies have pulled out hundreds of dependents of expatriate staff from the region.
Senior industry executives say the rising tide of violence could force them to withdraw from some areas completely.
Shell has already shut down its entire oil operation in the western side of the delta, a vast wetlands region in southern Nigeria, after a series of attacks in February which cut the OPEC nation's oil output by a fifth.