It said Khao Kho district jail is overcrowded and the refugees, who include women and children, had fallen ill because of lack of food.

"Non-governmental organisations [NGOs] and health authorities have been allowed to provide medical assistance to some of the refugees, but there appears to be no systematic and reliable provisions of food to all of them," Amnesty said in a statement on Friday.

"The refugees are still at risk of forcible return to Laos where they would be in danger of serious human rights violations, including torture and ill-treatment.

"Approximately two weeks ago, the provincial governor of Phetchabun, Torphong Ampan reportedly said that the refugees would be sent back to Laos via the Loei province border checkpoint.

"His deputy reiterated this statement, citing a National Security Council policy to deport illegal immigrants."

At least 6,000 ethnic Hmong Laotians live in the makeshift refugee camp in Huay Nam Khao. They started arriving there in large numbers in 2004, seeking refugee status. Some shifted there following the closure of the Hmong settlement at Wat Tham Krabok, near Saraburi - after the US took about 15,000 refugees.

Hundreds more arrived this year - many of them with alleged links to rebel groups still in conflict with Lao and Vietnamese troops in isolated parts of Laos. The majority of the new arrivals said they had been persecuted by the Lao military.

Local authorities have been threatening to return these people, but the Lao government has expressed doubt that they are Lao nationals.

Amnesty said: "The situation at the camp is gradually worsening and an increasing number of refugees are falling ill due to a lack of food.

"Warnings about the health of the camp population have been raised since March, when the police set up a checkpoint to restrict movements for the refugees and denied them the opportunity to search for food or firewood outside the camp."

The crisis at Phetchabun has been amplified by reports of continuing killings of Hmong in the Xaisomboon special region in central Laos.

On June 18, Nhia Tou Lor, a man in his seventies, was reportedly shot dead by a Lao soldier in Tong Khoun village, not far from Muang Cha.

Ten days earlier, another man called Xiong Pao Xiong was allegedly killed in the same area. Rights activists in the US said the killing was supposedly related to an ongoing investigation into a relative who fled to Thailand a year ago.

There have also been reports a few months ago of fighting in the jungles near Phu Pha Thi and Phu Bia.