Indonesian police are on terrorist alert following threats made by extremist Islamist group Isis (now known as 'Islamic State') to attack the Borobudur Temple cultural heritage site.

Security has been tightened around the Unesco World Heritage Site in Magelang, Central Java, after the Islamic State posted a threat on Facebook last Friday.

"God willing, (Borobudur) will be demolished by Islamic caliphate mujahidin," the statement read.

The statement also quoted an article from a radical website in which cleric Hartono Ahmad Jaiz condemned several statue construction projects around Indonesia as un-Islamic.

Central Java Police are scouring internet data for possible clues about the potential terrorist attack.

National police spokesman Ronny F Sompie told The Jakarta Post: "The Central Java police chief has ordered an investigation into the threat, and we expect cooperation from the temple's security personnel and the Indonesian Military, to safeguard the temple from any possible damage."

Temple security increased

Major-General Sunindyo, the commander of the Diponegoro Military Command, added that he was on standby to bolster security around the temple.

Marsis Sutopo, head of the Borobudur Conservation Agency, said the body has increased the number of security personnel in and around the temple.

"We've increased the number of security officers and have coordinated with PT Taman Wisata Candi Borobudur (which manages tourism activities at the temple) and the local police," he said.

The Borobudur Temple was built in the ninth century and is regarded as one of the world's largest and greatest Buddhist monuments.

In 1985, the relic was targeted by extremists, who detonated bombs at the site, damaging nine stupas: commemorative monuments which house scared relics associated with Buddha or other religious figures.

Last August, officials tightened security around the Borobudur Temple after the Buddhist temple Ekayana Graha Vihara was bombed in Jakarta.