The incident brought to seven the number of ships attacked in the Gulf of Aden in the past month, said Noel Choong, head of the Kuala Lumpur-based International Maritime Bureau’s (IMB) Piracy Reporting Centre.

The Japanese-operated cargo ship travelling from Singapore was sailing through the area on its way to the Middle East when at about 0808 GMT pirates in two speedboats opened fire at the vessel in a bid to stop it, Choong said.

The ship increased its speed and took evasive action, managing to escape the attack with no injury to the 20 crew, he added. There were no Japanese nationals on board.

Choong said the pirate mothership was spotted near the scene of the attack.

"We call on the United Nations and the international community, especially those with naval assets to take action," he told AFP.

"It appears the attacks are continuing and the pirates are looking for more ships to hijack," he added.

On Thursday, three ships - German, Iranian and Japanese - were hijacked by pirates off the Somali coast.

The waters off Somalia and Nigeria are the most pirate-infested in the world, with the IMB reporting 24 attacks in Somalia and 18 in Nigeria between April and June this year.

Of the 24 Somali attacks, 19 occurred in the Gulf of Aden off the country’s north coast.

On Tuesday, a Malaysian-registered tanker laden with palm oil and heading from Indonesia to the Dutch port of Rotterdam was also seized by pirates.

Last week, a Thai cargo ship was hijacked and a week before that, a Singapore-flagged vessel was attacked by pirates who fired a rocket-propelled grenade that landed on the ship but did not explode.

The series of attacks began on July 20 when pirates seized the Stella Maris, a Japanese-owned bulk carrier, and demanded a ransom from the owners.