Pirates have attacked a Chinese-owned bulk carrier and a Singapore-owned oil tanker in the Singapore Straits in the latest in a series of raids in the area, a maritime official said today.

A gang of pirates in small boats attempted to board the 38,172 tonne bulk carrier early on Friday morning, Noel Choong, head of the Piracy Reporting Centre of the International Maritime Bureau, told AFP .

"More than 12 boats surrounded the ship that was underway and the pirates attempted to board from both sides," he said.

The captain took evasive measures including spraying water from fire hoses, sounding the alarm and increasing speed to beat off the attack, he said. The pirates abandoned their attempts to board after pursuing the ship for 50 minutes.

Later Friday, pirates boarded the "Kyosei Maru" in Tanjung Pelepas port on the straits in Johor.

"Pirates armed with machetes tied up the crew and robbed them of their cash and valuables," Choong said.

The raiders, who were masked and are believed to be Indonesians, then escaped by boat, he said.

Recent surge in raids

The latest attacks follow five pirate raids on ships in the Malacca and Singapore Straits since Feb 28.

"We are very concerned about the recent surge in attacks in the Malacca and Singapore straits," Choong said.

"We hope Indonesia, Malaysia and Singapore will beef up patrols in these areas."

The three nations last year began coordinated patrols in the Malacca Straits, which leads into the Singapore Straits and forms a vital trade route from the Indian Ocean to the South China Sea.