Al Jazeera's Step Vassen, reporting from Jakarta, said the authorities were being tight-lipped about the operation but it appeared that police were hunting about 60 or so alleged militants.

The government is sending reinforcements to beef up special forces in Aceh [EPA]

Thursday's protracted gun battle involved heavy fighting, our correspondent said, and government reinforcements were being sent from Jakarta, highlighting the difficulty they were having getting the situation under control.

Rebel forces in Aceh province fought for independence from Indonesia for 30 years until a peace accord was signed in 2005.

Our correspondent said the rebel Free Aceh Movement no longer exists as it had joined the government following the peace pact, but it was possible a few former rebels had, after many years of rebuffing foreign militant movements, made contact with foreign fighters who had come to Aceh.

Police, who say they have found a paramilitary training camp and extremist literature, are still investigating whether there are links with al-Qaeda or other foreign fighter groups, she said.

Major-General Edward Aritonang, a police spokesman, said the 14 suspects charged on Thursday were caught in several raids carried out since February 22, when the first four were arrested by police after a gun battle in a suspected training camp in Aceh's mountains.

They confessed to undergoing paramilitary training, including weapons use and hand-to-hand combat at the raided camp in preparation for an attack, the spokesman said, but declined to specify the alleged target.

Under Indonesia's tough counterterrorism laws enacted in 2003, a conviction for planning a terrorist attack can carry a maximum prison term of 20 years.

Shipping lane threat

General Bambang, the national police chief, also said that police were investigating whether the fighters were linked to a "terrorist threat" to shipping in nearby Malacca Strait.

Indonesia, along with Singapore and Malaysia, stepped up maritime and air patrols in the waters this week after receiving a warning that an unnamed "terrorist" group was planning attacks on oil tankers in one of the world's busiest shipping lanes.

The International Maritime Bureau, a maritime watchdog, said it had urged ships in the region to keep a "strict watch" after being informed of a "terrorist threat" affecting the Malacca Strait from a foreign government agency.