Soldiers stopped cars at checkpoints and arrested about 60 more people in the third day of a crackdown on militants in Nigeria's volatile oil region on Sunday.
"We want to sanitize the state. We are not in any hurry to relax what we are doing," army spokesman Major Sagir Musa said. "If security reports require it, we will move to the creeks."
The sprawling delta includes a hard-to-police network of creeks and waterways.
Musa said 160 people have been arrested so far, though 30 of those have been released. He had claimed about 100 arrests on Saturday. Attempts by local human rights organizations and journalists to see the prisoners were unsuccessful.
The arrests are a reaction to a spate of kidnappings that has seen 15 foreigners snatched in the past three weeks. Ten have been released unharmed.
The kidnappings have severely curtailed movements of expatriate staff working for the oil companies that normally pump 2.6 million barrels of oil per day from the Niger Delta. A quarter of this production has been shut down by militant attacks earlier this year.
An e-mailed statement by a group calling itself the Movement of the Niger Delta People said that the last man released, a German freed on Friday, was let go on compassionate grounds.
The group had earlier sent photos of the hostage with demands that imprisoned tribal leaders be set free. It was the first appearance by the group, a common occurrence in a region where militant affiliations are constantly being reinvented and renamed.
"Our struggle is a just struggle against an oppressive and tyrannical regime," the statement said. "The resistance will continue."
Inhabitants of delta communities -- where many subsist on less than US$2 a day despite the hundreds of billions of dollars generated by oil exploration in the region -- have repeatedly detained foreign workers to protest living conditions.
Oil spills often pollute farms or rivers, which are the traditional means of sustenance, and government corruption means that the region is severely underdeveloped, lacking roads and reliable electricity.
However, the majority of the kidnappers who have taken foreigners in the past few weeks have not bothered to issue a list of demands, leading to speculation that kidnapping has become more of a commercial than a political activity.
After four foreigners were snatched from a nightclub in the city center last Sunday, Nigerian President Olusegun Obasanjo vowed to crack down on militant activities.
The operation has been met with skepticism by local residents.
"We need peace, food and shelter, not soldiers on our streets," said Fyneface Aaron, a 61-year-old water vendor.