Speaking in Singapore, a top State Department official said on Thursday Washington had good reason to believe that terror groups would target economically critical shipping lanes such as the Malacca and Singapore Straits.

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

The United States had raised its fears with Asian governments for the past year but remained deeply concerned about the safety of the region's shipping arteries, Daley said.

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

"We believe it essential to work with the countries of the region to rapidly improve maritime security," Daley said.

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

"Those where there are a number of Americans or American companies have to be alert to the possibility of terrorist attacks," Mueller said.

He said economic hubs such as Hong Kong also had to be alert to attempts by terror groups to abuse sophisticated financial systems for their own ends.

"Hong Kong is one of the principal transit points in the world. People come through, monies comes through, and it's important that...persons be alert to the abuse of the systems for terrorism, organised crime or other threats," Mueller said.

An attack on the Straits of Malacca would strike at Asia's economic heart.

More than a quarter of the world's trade, half its oil and much of its liquefied natural gas pass through the strait, which divides Indonesia, Singapore and Malaysia.

Piracy and armed robbery in the Straits of Malacca and Singapore had already increased rapidly, Daley noted.

"We know that terrorists in Southeast Asia are increasingly turning to soft targets. Moreover, as both the physical and political space in which they find sanctuary shrinks -- as the noose tightens -- we have good reason to believe terrorists will increasingly turn to the most unregulated of spaces -- the sea."

The U.S. warnings coincided with a threat by an hitherto unknown group to attack Asia-Pacific countries that have backed the war on terror launched by Washington in response to the September 11, 2002, attacks on the United States.

South Korea's embassy in Bangkok had received a threatening letter, a Thai police general said, while Pakistan's envoy to Thailand said his embassy had received a similar letter.

South Korea's Yonhap news agency said the letter, from a group calling itself the 'Yellow-Red Overseas Organisation', threatened attacks on major facilities in Australia, Japan, Kuwait, Pakistan, the Philippines, Singapore, South Korea and Thailand, between April 20 and 30.