Thai authorities have beefed up security in parts of the capital and other areas popular with tourists after the United States and Israel warned of a possible terrorist attack.

The army was helping the police in providing security, army chief Prayuth Chan-ocha told reporters on Saturday. People must be on alert and report anything suspicious, he said.

National Security Council secretary-general General Vichien Potphosri said special measures had been put in place in risk areas including Khao San Road, an area popular with backpackers, and parts of the main Sukhumvit road.

There's still nothing too worrying ... But we need to be on our guard, he was quoted as saying on Thairath newspaper's website.

The authorities arrested a Lebanese suspect this week after being warned by Israel of a possible attack in Bangkok, Deputy Prime Minister Chalerm Yoobamrung said on Friday, adding he was confident the situation was under control.

Television channel TNN reported that security had also been tightened in the northern town of Chiang Mai and the island of Phuket, both resort areas.

Israel's Counterterrorism Unit issued a severe travel warning to nationals on Friday, advising them against travel to the Thai capital in the near future.

The U.S. embassy in Bangkok told its citizens to be careful in areas of the capital frequented by tourists because of the threat of a possible attack by foreign terrorists. It declined to elaborate on the threat.

A Thai defence ministry source said Israeli intelligence had contacted Thai officials on December 22 with information that two or three suspects could be planning an attack in Thailand. However, the individuals travelled to the south and left the country.

The Israelis alerted Thai officials again on January 8 to the danger of an attack around January 13 to 15 in areas where there are often large concentrations of Western tourists.

The arrest was made after the second Israeli warning, the source said, adding that security officials were working closely with the United States and Israel.

Thai officials have said the suspect has links with Hezbollah, a Shi'ite Islamist group in Lebanon backed by Syria and Iran that is on the official U.S. blacklist of foreign terrorist organisations.

(Reporting by Viparat Jantraprap and Kochakorn Boonlai; Editing by Alan Raybould and Jonathan Thatcher)