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A group of Vietnamese asylum seekers have caused a security scare by climbing aboard an oil and gas support vessel off WA's North West.

It is believed the crew of the Offshore Quest barricaded themselves in a strongroom of the ship after discovering the group aboard on Wednesday night, fearing they could be pirates.

The crew radioed Customs, who immediately sent a patrol boat to assist.

Customs officers found 11 asylum seekers and one Indonesian crewman sitting quietly on the deck of the vessel.

It is thought their boat was tied up alongside the Offshore Quest, but had drifted away by the time the patrol boat arrived.

Customs officers are investigating whether the asylum vessel had become unseaworthy.

The incident was about 170km south-east of Ashmore Reef in an oil and gas-producing area.