The news came as Thai Prime Minister Thaksin Shinawatra was to meet with President Bush in Washington to discuss the "war on terrorism."
Arifin bin Ali, 42, also known as John Wong Ah Hung, was arrested in Bangkok on May 16 following a tip by Singapore's intelligence agency, the Home Affairs Ministry said in a statement issued late Tuesday.
"Arifin has disclosed to the ISD that he is involved with a group of like-minded individuals in planning terrorist attacks against certain targets in Thailand," the statement said.
The Singapore embassy and four other embassies in Bangkok were targeted in the attacks, the statement said. It did not name the other diplomatic missions.
Singapore authorities have conveyed the information to their counterparts in Thailand, the statement said. Both countries are working together in a joint investigation, it added.
Arifin was handed over to Singaporean authorities a day after his arrest, the statement said. It did not explain why the announcement of the arrest was delayed.
It said Arifin was an instructor in Jemaah Islamiyah's military training and security unit. He was trained to use weapons and explosives at a camp at Mindanao in the southern Philippines run by the Moro Islamic Liberation Front, an Islamic separatist group, it added.
He fled to Malaysia in December 2001 when the government began a crackdown on Jemaah Islamiyah that lead to the arrest of 31 suspected terrorists, the ministry said. Jemaah Islamiyah has been accused of being responsible for bomb attacks on the Indonesian island of Bali last October that killed 202 people, mostly foreign tourists.
Arifin is being held under Singapore's Internal Security Act, which allows indefinite detention without trial, the statement said.
Singapore officials last year uncovered an apparent plot to attack embassies of Australia, the United States and Israel.
Thai police earlier Tuesday said they had arrested three Muslim religious teachers suspected of having links with Jemaah Islamiyah, a regional terrorist network.
"We have evidence to prove that they are members of JI and that they were plotting to conduct violence in the country, but we don't have details of which targets," said Maj. Gen. Chumpon Manmai, commander of the Special Branch police, which made the arrests.
He said he could not confirm some reports that the three men were planning to attack Western embassies. Thai television station ITV identified the three men as Maizuru Haiji Abdullah, Mujahid Haiji Abdullah and Dr. Waehamadi Wadao.
The Jemaah Islamiyah is blamed for last year's bombings in Bali that left more than 200 people dead, mostly Westerners. The group is believed to be linked to Osama bin Laden's al Qaeda network.
Last month, the Cambodian government broke a Muslim group with the help of the United States, and charged three of its members — two Thais and an Egyptian — with international terrorism. They were also suspected to have links with Jemaah Islamiyah.
Shinawatra recently said the regional terror group has inactive members in Thailand.
A U.S. Embassy spokesman confirmed the arrests, but said he had not details.