Thai police charged a Lebanese man suspected of planning an attack in Bangkok after they raided a property on Monday and discovered chemicals that could be used to make a bomb.
The United States has warned of a "serious" threat of a terrorist strike on tourist areas in Bangkok and a manhunt is under way for a second suspect who could be hiding in the Thai capital, prompting extra security.
The Lebanese man is accused of breaking weapons control law, deputy national police chief Pansiri Prapawat said, after a large amount of fertiliser and ammonium nitrate was found at an address rented by the suspect.
"We will conduct further investigation into whether he violated any other laws," he told reporters.
Ammonium nitrate is commonly used in agriculture, but mixed with other substances can make a bomb. Possession of the chemical requires a permit in Thailand.
Security has been stepped up around possible targets in Bangkok, including the Israeli embassy and the Khao San Road backpacker district, and at Suvarnabhumi airport, Thailand's main air hub.
Prime Minister Yingluck Shinawatra again reassured the public that the situation was under control.
"We continue to keep a close watch and we have also increased the security presence," she told reporters.
Thai authorities allege the Lebanese man, who was detained on Thursday, has links to Hezbollah, an Iranian- and Syrian-backed Muslim Shiite group that is blacklisted as a terrorist organisation by Washington.
When contacted, Hezbollah declined to comment on the case.
The suspect, who is also believed to have a Swedish passport, was detained based on intelligence provided by Israel, according to Thai officials.
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Police said Saturday the suspect had admitted during interrogation that a group was planning an attack in Thailand, but that the plot had been called off.
Israel would not confirm or deny a role in the arrest, but has urged its citizens to avoid Bangkok because of "a serious and imminent danger".
Thai authorities had repeatedly played down the risk and withheld information about any possible plot until after the United States warned Friday of a terrorist threat against tourists in the kingdom.
"Foreign terrorists may be currently looking to conduct attacks against tourist areas in Bangkok in the near future," the US embassy in Bangkok said in an emergency message posted on its website.
US Ambassador Kristie Kenney wrote on Twitter on Monday that the US advice "still stands."
She has previously described the threat as "credible, serious and specific."
Foreign Minister Surapong Tovichakchaikul has expressed "disappointment" that the United States did not consult Thailand before issuing the alert.
The safety scare is another blow to the kingdom's tourist-friendly image, which was badly dented last year by devastating flooding across much of the country, as well as rounds of rival political protests in recent years.
In 2008 a nine-day blockade by protesters of Suvarnabhumi airport stranded tens of thousands of travellers, and the industry was badly hit again in 2010 during anti-government street demonstrations.
Bangkok's central shopping area was reduced to a battle zone during an army crackdown on the 2010 rallies, and more than 90 people were left dead.
An eight-year shadowy insurgency continues to plague the country's Muslim-majority deep south, but the rebels have never been known to attack outside the region.
In the most prominent terrorist attack in Southeast Asia in recent history, 202 people -- most of them foreign tourists -- were killed in 2002 by bombings on the Indonesian island of Bali.