Jakarta – Five human rights workers and another man are "at grave risk" of torture and ill-treatment after being arrested by police in Aceh province, Amnesty International said Tuesday.

The London-based human rights group said the five are believed to have been arrested "solely for their peaceful expression of their political beliefs and their work in defence of human rights."

Police and troops are in the 10th month of a major offensive aimed at crushing separatist rebels in the province on Sumatra island.

Amnesty said in a statement the incident began with the arrest on February 19 of a man called Masrizal on suspicion of being a Free Aceh Movement (GAM) member.

Three days later a rights worker called Iwan Irama Putra, an acquaintance of Masrizal, was also detained.

It named the other detained rights workers as three women called Harlina, Nova Rahayu and Nursida and a man called Syafruddin. Harlina and Syafruddin were reportedly beaten before being taken away.

"Grave human rights violations, including extrajudicial executions of both adults and children," have been reported since the latest offensive was launched last May, Amnesty said.

Amnesty said members of political organisations campaigning for Aceh's independence and human rights activists are among those held.

"Anyone detained by the police and military in (Aceh) is at risk of torture, ill-treatment and other human rights violations," it said.