Pirates have boarded a Liberian-flagged oil tanker with 16 crew members in West Africa's Gulf of Guinea, the Danish vessel owner said Tuesday.

The attack took place southwest of Port Pointe-Noire, Congo.

The Monjasa Reformer "experienced an emergency situation" late Saturday, Monjasa, the Denmark-based company that owns the ship, said in a statement.

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The crew sought refuge in a citadel — a safe area on the ship — in line with the on board anti-piracy emergency protocol, said company spokesman Thorstein Andreasen.

The nationalities of the crew members and the pirates were not immediately known. Communication channels with the ship are down and international authorities have been alerted, said the statement.

The exact location of the tanker is not known. The last position was transmitted Sunday without any updates since, said Martin Kelly, senior analyst with the London-based EOS Risk group, a security company operating in the area.

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The Gulf of Guinea is the world’s most dangerous spot for attacks on ships. In June, the United Nations Security Council unanimously adopted a resolution strongly condemning piracy, armed robbery and hostage-taking in the area. This hijacking took place further south in an area that is not typically attacked by pirates.

"This is worrying since it’s rare in this area compared to the Gulf of Guinea, for example, where multiple ship hijackings take place every year," said Rida Lyammouri, senior fellow at the Policy Center for the New South, a Moroccan-based think tank.

"Hopefully we are not witnessing a new trend and (this) is just an isolated incident. This also could be explained by increased security measures in the Gulf of Guinea and pirates are looking into new areas of operations," he said.