Exxon Mobil Corp. units in Nigeria declared force majeure on oil loadings from multiple terminals in the country following industrial action, a spokesman for one of the firms said.

Three units of the US major declared the step, which allows them to skip contractual obligations due to events outside of their control, a spokesman for one of those entities, Mobil Producing Nigeria Unlimited, said by email.

Exxon is among the largest oil producers in Nigeria and its Qua Iboe program is among the nation’s biggest export streams. The company pumped more than a fifth of the country’s barrels last year, according to data compiled by Bloomberg.

The disruption will be a blow to Nigeria, whose oil supply has been beset by disruptions, including theft from oil pipelines and sabotage, for several years. Traders can sometimes be wary of purchasing Nigerian barrels because of concerns the cargoes won’t load as planned.

The entities that declared force majeure were Esso Exploration and Production Nigeria Ltd., and Esso Exploration and Production (Offshore East) Ltd. declared the measure from their respective terminals, the spokesman said.