The city state, a long-time ally of Washington that hosts a US navy logistics base and numerous American companies, has detained 12 Singaporeans and one Malaysian for allegedly plotting to bomb the US embassy and other targets.

Malaysia also arrested dozens of people. The two nations say the suspects are part of Jemaah Islamiah, a Muslim group believed to have links to the al Qaeda network accused of carrying out the attacks on the World Trade Center and the Pentagon last year.

"We are maintaining a greater level of vigilance during this period," Soh Poh Theen, director of Singapore's National Security Secretariat, said in an email reply to a Reuters query.

Soh said there would be more joint patrols by the police and armed forces at Changi Airport, which handled about 28 million passengers last year.

Visitors to the American Club, a large social venue in the heart of Singapore, say they have noticed more armed guards in recent days.

The city state tightened security at all entry points and stepped up cargo and immigration checks immediately after the attacks on the United States.

"The situation is being assessed constantly," a spokesman from the Ministry of Home Affairs said. "Should there be a need, the level of vigilance can be stepped up further."

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