Gunmen killed two policemen in Nigeria's oil-producing Niger Delta as they slept in a security post on the evening of May 9 and three soldiers were killed in a second attack, Nigerian police and the army said.

The raids took place a day after five police officers were shot dead in the same region.

Recent violence has raised concern that militants might resume an insurgency that has been quiet for the past several years. A labor union on May 10 called for the evacuation of oil workers from the region.

The two policemen, who were asleep on guard duty, were killed in Delta's Rivers state, a police spokesman said. The soldiers died in an attack in Bayelsa state, the army said.

Last week, a group known as Niger Delta Avengers attacked a Chevron Corp. (NYSE: CVX) facility in the Delta after claiming a strike in February against a Royal Dutch Shell Plc (NYSE: RDS.A) pipeline, which shut down the 250,000 barrel a day Forcados export terminal. The violence has pushed Nigeria's crude output near to a 22-year low.

"Best thing for any reasonable company to do is evacuate its workforce," Cogent Ojobor, chairman of the Warri branch of the Nupeng oil labor union, said.

Chika Onuegbu, chairman of the trade union in Rivers state, said Chevron had evacuated some staff from the Delta following a similar move by Shell.

"There is high alert around various installation around the Niger Delta due to recent attacks," Onuegbu said. "Those evacuated are where their platforms have been attacked but others are working."

Ikeja Electricity, Nigeria's biggest power firm, said it expected extended outages after the attack on Chevron hit gas supplies needed to generate electricity.

Delta residents have long demanded a greater share of oil revenues. Crude oil sales account for about 70% of national income in Nigeria but there has been little development in the region.

President Muhammadu Buhari has extended an amnesty agreement signed with militants in 2009 to end their campaign to blow up pipelines, but has upset them by ending generous pipeline protection contracts.

The incidents are a further challenge for a government faced with an insurgency by the Islamist militant Boko Haram group in the northeast, and clashes between armed nomadic herdsmen and local people over land use in various parts of Nigeria.