Washington – Human rights groups on Tuesday voiced deep concern over the fate of a New York-based activist missing in Indonesia's Aceh province, and demanded more action from US officials on his case.

Jafar Siddiq Hamzah, a permanent resident of the United States and director of the New York-based International Forum on Aceh (AFA), vanished in the city of Medan on August 5.

His friends say they fear he may have been kidnapped because of his vocal campaign against killings and torture in Aceh, which has been consumed by violence linked to a drive for independence.

"We are not hopeful, we cannot say we are optimistic," said John Miller of the US-based East Timor Action Network, which has worked closely with Hamzah. "Frankly we believe the military is responsible," he added.

Senior officers have denied abducting Hamzah, but human rights groups here put little credence in their claims of innocence. "In the past, military denials have meant absolutely nothing," said Sidney Jones, Executive Director of Human Rights Watch Asia in New York. "We don't have the sense that the military is using its various contacts to find out where Jafar is."

The United States has expressed concern over Hamzah's disappearance and diplomats in Jakarta have been meeting Indonesian officials to try to trace him. But Miller called on the US government to do more, saying an expression of concern was needed from Secretary of State Madeleine Albright.

"We would argue that the United States should take it up a notch or two diplomatically," he said. "They need to say something much clearer out of Washington itself."

A State Department official said Tuesday that Washington had been "making queries" over Hamzah's welfare. "We continue to press them," he said.

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.

A student and part-time taxi driver, he left New York for Aceh in late June to set up the Support Committee of Human Rights for Aceh (SCHRA). The GAM has been fighting for an independent Islamic Aceh state since 1976 and more than 5,000 people have been killed in the fighting over the past decade.