Jakarta – The United States on Wednesday expressed "deep concern" over the sudden disappearance of a US-based human rights activist in the Indonesian city of Medan last Saturday.

A statement issued by the US embassy here said that the activist, Jafar Siddiq Hamzah, a permament resident of the United States and director of the New York-based International Forum on Aceh (AFA), was last seen in Medan on August 5.

"We understand that ... his family and friends have been unable to establish his whereabouts since he failed to appear for a scheduled meeting" the same evening, the statement said.

It said the embassy had made its concern known to the Indonesian government, police and military officials, and appealed for information on his whereabouts.

On Tuesday a member of Hamzah's family told AFP in the Acehnese capital of Banda Aceh that they feared he had been abducted. "Normally brother Jafar calls the family every two hours to inform us of his whereabouts," Hamzah's younger brother Jamaluddin said. "But since he finished meeting his friend at around 1am on Saturday, we have not heard from him. Our relatives in Medan have tried to look for him but we have not been able to find him. We are now really worried," he added.

Hamzah, a native of Lhokseumawe in North Aceh, campaigns for the redress of massive human rights abuses during 10 years of military operations against the Free Aceh (GAM) separatist rebel movement.

He returned to Aceh some two weeks ago to set up the Support Committee of Human Rights for Aceh (SCHRA), Jamaluddin said, adding that his brother had planned to stay in Aceh for one year.

Jamaluddin said he did not want to speculate as to Hamzah's possible kidnappers, but pleaded with any captors to release his brother. "What wrong has he done? [What] can cause his disappearance because his purpose of visiting Medan was only to meet old friends," Jamaluddin said.

Syarifuddin Bantasyam, the director of Forum of Human Rights Carein Banda Aceh, told AFP that Hamzah could have been kidnapped because of his vocal "international-level campaign" on killings and torture in Aceh.