About 100 people have been arrested in a military crackdown on suspected militants in Nigeria's restive, petroleum-rich south, an army spokesman said Saturday.

Shooting into the air, troops entered houses in a Port Harcourt neighborhood Friday, sending men and women into the streets screaming. Gunfire sounded for at least an hour. By Saturday morning, roadblocks had been set up around the city.

The operation was continuing in areas raided Friday, army spokesman Maj. Sagir Musa said, without elaborating on the arrests. He said he did not know of anyone seriously injured or killed in the crackdown.

"After a careful analysis of the security situation, we will swoop on the creeks by land and air," Musa said. Nigeria's southern delta contains a network of narrow creeks and waterways that can make policing difficult.

Port Harcourt, Nigeria's oil hub, has been rocked by a series of kidnappings — 15 in the past two weeks — that led President Olusegun Obasanjo to declare the clampdown earlier this week. Ten hostages have been released unharmed, but five remain unaccounted for. Hostages taken by militants looking for ransom are rarely harmed and most kidnappings end peacefully.

Poland's foreign ministry said Saturday that it was "making all effort" to secure the release of a Polish oil mining engineer kidnapped from a nightclub last Sunday. Poland's ambassador in Abuja, Grzegorz Walinski, told the Gazeta Wyborcza daily that Nigerian police known where the Pole and other foreigners abducted that night were being kept.

On Friday night, a German hostage was released to Nigerian authorities, Rivers State official Bletyn Wikina said. The man was taken Aug. 3 from a makeshift checkpoint by gunmen wearing military fatigues.

Wikina did not say if a ransom had been paid, and it was unclear if the release was linked to the campaign against militants. A German Foreign Ministry spokeswoman declined to comment on details of the release.

Also Friday, Port Harcourt's main airport closed, cutting off a major entry and exit point to the city for oil-industry workers. Officials cited a Thursday electrical fire for the closure.

Militant attacks have cut Nigeria's daily oil production by nearly a quarter from its normal 2.6 million barrels. The country is Africa's biggest oil exporter and the fifth-largest supplier of crude to the United States.

Many Nigerians in the region say the violence stems from popular discontent over the portion of Nigeria's oil earnings that returns to their areas.

"The likely consequences of this will be an escalation of the unspoken war in the Delta," said Oases Nosaze, head of the country's large civil society organization. He said his group has always been opposed to a military solution because "we believe the issue is a social and political one and that people have legitimate grievances."

The Abuja-based federal government apportions the earnings from Nigeria's oil industry. Despite the great riches, most of the Niger Delta is mired in extreme poverty, and most residents live without electricity, health care or schools. Many Nigerians accuse their leaders of stealing much of the oil funds. Nigeria is routinely ranked among the world's most corrupt nations.