The State Department issued a statement urging Americans in Southeast Asia "to remain vigilant with regard to their personal security and to exercise caution."
The bomb blast in the Indonesian resort of Bali killed more than 190 people, many of them Australian.
And the State Department warned: "In the aftermath of the terrorist bombings in Bali, Indonesia, the possibility exists that similar attacks may occur in other Southeast Asian nations.
"The Department is concerned that individuals and groups may be planning terrorist actions against United States citizens and interests, as well as sites frequented by Westerners," it said.
It added that extremist groups in the region "have transnational capabilities to carry out attacks against locations where Westerners congregate.
"Increased security at official US facilities has led terrorist groups and their sympathizers to seek softer targets such as clubs, restaurants, places of worship, schools, outdoor recreation events, hotels, resorts and beaches," the State Department said.
The statement highlighted that Jemaah Islamiyah (JI), an Indonesian group which the United States declared a "foreign terrorist organization" on Oct. 23 has "cells operating throughout Southeast Asia."
The State Department said that JI members detained in the region "have revealed links with al-Qaeda, other regional terrorist groups, and previous terrorist attacks in the region."
Confessed JI member Fathur Rohman al-Ghozi, an Indonesian who was convicted of illegal possession of explosives, revealed that al-Qaeda fighters have even conducted training sessions in the Philippines.
Al-Ghozi told authorities that arrested Kuwaiti terrorist Omar al-Faruq and the commander of the al-Qaeda camp in Afghanistan have both conducted terrorist training of Filipino Islamists in at least two camps of the separatist Moro Islamic Liberation Front (MILF).
Al-Ghozi admitted that he trained local Islamists, mostly MILF members, in bomb-making, weapons handling, surveillance, sabotage, sabotage and terrorist cell formation in at least two MILF camps for three years beginning in 1996.
Intelligence sources identified the camps as Abubakar in Maguindanao, which fell to government hands in 2000, and Camp Obaida in Saranggani.
Al-Ghozi admitted that he himself went through an indoctrination by Indon Muslim cleric Ababakr Ba’asyir, who was arrested recently by Indonesian authorities.
This was the same time that al-Faruq, who was arrested on June for his role in the church bombings in Indonesia on Christmas Eve of 2000, arrived in the country to conduct training in Camp Abubakar.
Al- Faruq, who is now detained by the United States in their air base in Bagram, Afghanistan, was accompanied by a certain al-Mughira al Gaza’iri, who is believed to be the commander of an al-Qaeda training camp in Khaldan, Afghanistan, the source said.
Al-Faruq was supposedly sent to Southeast Asia by no less Abu Zubaydah, supposedly a top associate of terrorist Osama bin Laden.
The source said the US Central Intelligence Agency (CIA) has also traced a cell phone number that Al-Ghozi supposedly dialed on his cell phone back to al-Faruq.
"Al-Ghozi carefully established the cell, opening bank accounts, networking and recruiting. He also learned the local language well enough that he almost passed himself off as a Filipino," the source said.
Al-Ghozi appeared to be so successful that Philippine cells became a major logistics center for acquiring explosives, guns and other equipment.
Local officials and their counterparts in other Southeast Asian nations believe that Al-Ghozi was responsible for the purchase of firearms which were later shipped to Poso, Indonesia but Malaysian authorities intercepted the shipment at Ambon in mid-2001.
Al-Ghozi also admitted that he was among those who planned the Dec. 30, 2001 bombings in Metro Manila that killed 21 people and injured a hundred others.
The bombing was supposedly part of an ambitious JI scheme to establish a pan-Islamic state in Southeast Asia, to be known as "Daulah Islamiah Raya," and would include all of Indonesia, Malaysia, Singapore, Brunei and Mindanao.
The JI is headed by Indonesian preacher Riduan Isamuddin, alias Hambali, a 36-year-old Indonesian peasant who supposedly coordinates all terrorist activities in Southeast Asia for the Al-Qaida terrorist network of Osama bin Laden.
Intelligence sources said the plot to establish Daulah Islamiah Raya was uncovered by US authorities after they captured a JI fighter in Aghanistan.
Information extracted from the unidentified JI fighter supposedly led to the arrest of 15 JI fighters who were planning to bomb a US ship and other facilities in Singapore.
The arrests in Singapore forced Al-Ghozi to flee to the Philippines where he was arrested on Jan. 15. His arrest led to the seizure of a ton of explosives inside a leased compound in General Santos City in South Cotabato. — With AFP report