Boats ferry passengers to big vessels waiting for docking at the Singapore waterfront. (AFP)

Bangkok, April 22 (Reuters): Letters sent to the South Korean and Pakistani embassies in Thailand threatening America’s Asian allies sparked fresh security fears today as Washington told the region to be on alert for attacks on its crowded sea lanes.

Speaking in Singapore, a top state department official said US authorities had good reason to believe terror groups would target critical shipping routes such as the Malacca and Singapore straits.

Seoul ordered an immediate extension of already heightened security measures at ports and airports in response to a letter signed by a hitherto unknown group calling itself the Yellow-Red Overseas Organisation.

“The letter said eight countries with alliances with the US were being targeted,” a foreign ministry official said, without elaborating.

Seoul’s Yonhap news agency identified the countries as Australia, Japan, Pakistan, the Philippines, Singapore, South Korea, Thailand and Kuwait, and said the threat was made against “major facilities” from April 20 to 30.

All the countries have to a greater or less extent helped Washington in its “war on terror”. Seoul, which has had 600 military engineers and medics in Iraq since last May, plans to send another 3,000 troops by mid-June. An official in charge of port security at the maritime affairs and fisheries ministry said the National Intelligence Service had ordered the alert until April 30.

Apart from the threats to ports such as Pusan, South Korea’s second city and one of Asia’s busiest ports, US authorities now believe the region’s shipping lanes, the arteries of world trade, could be the militants’ next target.

“We have begun to focus on the potential of a disastrous maritime terrorist incident,” Matthew Daley, US deputy assistant secretary of state, told a security conference in Singapore.

An attack on the Strait of Malacca would strike at Asia’s economic heart. More than a quarter of the world’s trade, half of its oil and much of its liquefied natural gas pass through the strait, which divides Indonesia, Singapore and Malaysia.

The US had raised its fears with Asian governments for the past year but remained deeply concerned about the safety of the region’s shipping lanes, Daley said.

Assaults by al Qaida on commercial shipping in Yemen and the Arabian Sea and planned attacks in several straits, including the Strait of Gibraltar between Spain and North Africa, showed that US concerns were not simply theoretical.

In Hong Kong, FBI director Robert Mueller said the city and commercial centres like it could be targets for al Qaida and other terror groups seeking to inflict economic damage.