Reiterating its concern about the continuing secrecy surrounding these detentions, the organisation said, 'It is vital for the government to release more information, and to be publicly accountable for who is being held and in what circumstances.The authorities must also ensure that the basic rights of all those detained are respected.'

Amnesty International cited - among others - the case of four South Asian men detained in Connecticut 10 days ago, who have had no access to their families since their arrest. When their attorney tried to visit them, officials would not confirm they were detained and their names did not appear on Immigration and Naturalization Service (INS) computerized lists; he gained access only after obtaining their alien numbers through another source.

Last week the US Attorney General released partial data on those detained after 11 September, revealing that 104 individuals have been charged with various federal criminal offences, around half of whom are still in custody. The names of 93 have been made public, while the others remain sealed under court orders. 548 other - as yet unidentified - individuals remain in custody on immigration charges.

'The information provided by the Attorney General leaves many questions unanswered,' Amnesty International said, stressing that no details were given, for example, on where detainees are held and how long they were held before being charged. The list also left more than 400 individuals unaccounted for, based on prior government figures of more than 1,180 arrested or detained. 'They may have all been released or deported, but no-one seems to know except the government,' the organisation said.

Although INS guidelines state that INS detainees should be told of organisations able to provide free legal advice, this does not always happen in practice. 'Unless the detainees have access to telephones and know who to contact they could simply disappear into the system,' Amnesty International said.

Last week, Amnesty International presented a detailed memorandum to the Attorney General, in which the organisation outlined a range of concerns relating to the post-11 September investigations. These included reports of incommunicado detention ; harsh conditions and ill-treatment of Muslim detainees in custody; concern at government powers under the recent legislation to detain non-nationals indefinitely on the basis of mere suspicion of involvement in 'terrorism'; and the use of secret evidence.

The memorandum also addressed speculation over the use of torture or 'pressurized' interrogation techniques; the situation of suspects arrested abroad and trials by military commissions. Amnesty International recently called for revocation of the order to establish such commissions as they would set up an intrinsically discriminatory parallel justice system which does not comply with internationally recognised fair trial standards.

Several recent cases further highlight Amnesty International's continuing concerns. These include:

Mazen Al-Najjar, a Muslim cleric, was arrested last month on a deportation order for overstaying his student visa. Despite having no criminal record, he is locked down for 23 hours a day in a small cell in solitary confinement in a maximum security prison and is denied visits from his family. Dr Al-Najjar was previously detained in a US jail for more than three years on the basis of secret evidence, while appealing against the deportation order, but a federal judge ordered his release last December, after finding there were 'no bona fide reasons to conclude that [he] is a threat to national security'. He his currently held in far worse conditions than before - with no prospect of release because, as a stateless Palestinian, he has no country to return to.

A Yemeni student arrested on a visa violation after several years in the USA, has reportedly agreed to be deported to Yemen - despite fearing he would be at risk there - after spending 45 days in solitary confinement in a high security unit.

In recent weeks, Amnesty International has repeatedly expressed its concern about different aspects of the USA's response to the 'terrorist threat'. These include the use of military tribunals to try foreign suspects and fears that - in the face of the refusal by many countries to extradite people to the USA on the grounds that they may face the death penalty - the US government may resort to tactics, which in the past have included the use of abduction, to circumvent extradition protections.

'For justice to be done - and to be seen to be done - the US government must abide scrupulously to the highest standards of legality and transparency,' Amnesty International said.