Shell workers at Nigeria's Bonga oil field in the southern Niger Delta are being evacuated following a threat from militants, a senior labor union official said on May 9.

"We are aware of the development and the evacuation is being done in categories of workers and cadres," Cogent Ojobor, chairman of the Warri branch of the Nupeng oil labor union, said. "My members are yet to be evacuated."

He did not say how many workers in total were being evacuated, nor how many members of the Warri branch of the Nupeng union were waiting to be evacuated.

Shell said earlier on May 9 that oil output was continuing at its oil fields in Nigeria despite local media reports of a militant attack near its Bonga facilities.

"Our operations at Bonga are continuing," a spokesman for Shell Nigeria Exploration and Production Co. (SNEPCo) said in a statement. It said it would continue to monitor the security situation in its operating areas and take all possible steps to ensure the safety of staff and contractors.

Last week, militants attacked a Chevron Corp. (NYSE: CVX) platform in the Delta where tensions have been building up since authorities issued an arrest warrant in January for a former militant leader on corruption charges.

Nigerian President Muhammadu Buhari has said there would be a crackdown on "vandals and saboteurs" in the Delta region, which produces most of the country's oil.

A group known as the Niger Delta Avengers claimed responsibility for the Chevron attack. The same group has said it carried out an attack on a Shell oil pipeline in February which shut down the 250,000 barrel a day Forcados export terminal.

Residents in the Delta have been demanding a greater share of oil revenues. Crude oil sales account for around 70% of national income in Nigeria but there has not been much development in the poor Delta region.

Buhari has extended a multimillion-dollar amnesty signed with militants in 2009 but upset them by ending generous pipeline protection contracts.

The militancy is a further challenge for a government faced with an insurgency by the Islamist militant Boko Haram group in the northeast and violent clashes between armed nomadic herdsmen and locals over land use in various parts of the country.