Royal Dutch Shell Plc has unexpectedly shut a processing unit at its 500,000 barrels-per-day (bpd) Singapore refinery, trade sources said on Monday.

A power failure likely caused the outage, leading to the flaring of gases at the unit, one of the sources said. Black smoke was seen emerging from the site, the sources added.

It was not clear which unit was shut and Shell was not immediately available for comment.

The Singapore Civil Defence Force (SCDF) said it received a call at 10 a.m. local time (0200 GMT) about a fire alarm on Pulau Bukom, where the Shell refinery is located.

SCDF resources were dispatched. However our services were not required as the incident was already resolved prior to our arrival, the SCDF said in an e-mailed statement.

Shell operates three crude distillation units, a condensate splitter, a fluid catalytic cracker, a hydrocracker and an ethylene cracker at the Bukom site.

The entire refinery shut down in September last year after a fire struck the complex.

(Reporting by Seng Li Peng, Luke Pachymuthu, Jasmin Choo, Jessica Jaganathan and Florence Tan; Editing by Himani Sarkar and Joseph Radford)