The Chevron oil company has reopened two oil stations in Nigeria's Niger Delta region under army protection.
Most of Nigeria's oil comes from the Niger Delta but its people are poor
They were closed last week after attempts by a local militia group to sabotage oil facilities.
The Niger Delta People's Volunteer Force had issued the threats in protest at the detention on Tuesday of their leader Mujahid Dokubo-Asari.
The group seeks self-determination for the oil-producing region, which remains poor and under-developed.
On Sunday, Mr Asari's lawyer, Uche Okwukwu, said the militia leader had called for calm.
"Asari gave me the instruction to tell them that nobody should harm any foreigner or do anything criminal," Mr Okwukwu told Reuters news agency.
"They shouldn't do anything that could put his case in jeopardy. It's not in the interest of the movement," Mr Okwukwu added.
The lawyer was released on Friday after being arrested when he went to defend Mr Asari.
Alali Horsfall, deputy leader of the militia group, told the AP news agency that the group had "suspended action".
Mr Asari was arrested on Tuesday, and on Thursday was remanded in custody for two weeks by a judge in the capital, Abuja.
Police are investigating charges of sedition in connection with a newspaper interview in which Mr Asari allegedly called for the break-up of Nigeria.
On Thursday, more than 100 armed men in boats stormed the Idama flow-station, sources close to the Chevron oil company said.
The Robertkiri station was later closed following threats. Together, they account for less than 1% of Nigeria oil production of 2.5m barrels a day.
The Niger Delta remains one of Nigeria's poorest and least developed regions, although it accounts for most of the oil produced by Nigeria, Africa's largest oil-producer.