ExxonMobil subsidiary Mobil Producing Nigeria has declared force majeure on exports of Nigeria’s Qua Iboe crude oil, the country’s largest export stream, a spokesman said on Friday.

The declaration came after the company observed a “system anomaly” during a routine check of its loading facility on July 14.

“We are working to ensure loading activities at the facility return to normal. We cannot speculate on any timeline for repairs,” the spokesman said. “Qua Iboe Terminal is operating and production activities continue.”

Nigeria has struggled to maintain its crude oil production following a spate of militant attacks and technical problems that in May pushed production briefly to 30-year lows. While the cause of the latest issue was not immediately clear, traders said it would take least two to four weeks to repair.

Earlier this week, Exxon denied claims from the Niger Delta Avengers that the militant group had blown up the Qua Iboe 48″ crude oil export pipeline operated by the company.

Spokesman Todd Spitler said on Friday there was no connection between the force majeure and militant attacks.

The Avengers, a group that has carried out attacks on oil facilities in Nigeria’s southern Niger Delta energy hub in the last few months, has said in statements on its website that Exxon lied when it denied that an attack had taken place.

In a statement posted on its website late on Friday, it warned the company not to carry out any repairs.

“If ExxonMobil fails to listen to us (Niger Delta Avengers), your personnel are going to be our next casualties not pipelines,” said the group.

The Avengers, whose attacks briefly pushed Nigeria’s oil production to 30-year lows in the spring, has said it wants a greater share of the country’s oil wealth to go to the impoverished region that is the main source of crude.

Exxon has struggled to bring production of Qua Iboe back to normal after an accident in May on a drilling rig that damaged a pipeline, after which the company also declared force majeure.

Since lifting that declaration in early June, there have been roughly three revisions to loading schedules, attributed to a slower-than-expected resumption of flows, with loading delays of at least five days.