The British government is under pressure to take up the case of Bradley Manning, the soldier being held in a maximum security military prison in Virginia on suspicion of having passed a massive trove of US state secrets to WikiLeaks, on the grounds that he is a UK citizen.

Amnesty International called on the government to intervene on Manning's behalf and demand that the conditions of his detention, which the organisation calls "harsh and punitive", are in line with international standards.

Amnesty's UK director, Kate Allen, said: "His Welsh parentage means the UK government should demand his 'maximum custody' status does not impair his ability to defend himself, and we would also like to see Foreign Office officials visiting him just as they would any other British person detained overseas and potentially facing trial on very serious charges."

Clive Stafford Smith, director of Reprieve, which provides legal assistance to those facing capital punishment and secret imprisonment, likened the conditions under which Manning is held to those in Guantánamo Bay.

"The government took a principled stance on Guantánamo cases even for British residents, let alone citizens, so you would expect it to take the same stance with Manning."

Manning is a UK citizen by descent from his Welsh mother, Susan. Government databases on births, deaths and marriages show she was born Susan Fox in Haverfordwest in 1953.

She married a then US serviceman, Brian Manning, stationed at a US base near the city, and they had a daughter, Casey, in the same year. Bradley was born in Oklahoma in 1987.

Born in the US, he is a US citizen. But under the British Nationality Act of 1981, anyone born outside the UK after 1 January 1983 who has a mother who is a UK citizen by birth is British by descent.

"Nationality is like an elastic band: it stretches to one generation born outside the UK to a British parent," said Alison Harvey, head of the Immigration Law Practitioners' Association in London.

So far, however, Manning's British status has not impinged itself upon the UK authorities. The British embassy in Washington said it had not received any requests to visit Manning in jail. "It hasn't crossed our path yet," an official said.

The issue of the soldier's nationality has been bubbling furiously on Twitter in recent days and has been taken up by the UK branch of the Bradley Manning supporters network.

He has been held in the brig of Quantico marine base in Virginia since last July, having been arrested in Iraq, where he was stationed as a US army intelligence analyst two months previously.

He is alleged to have been the source of several WikiLeaks exposés of US state secrets, including the massive trove of cables released in November.

He has been charged with illegally obtaining 150,000 secret US government cables and handing more than 50 of them to an unauthorised person. Yet campaigners say the conditions in which he is being held are wholly disproportionate.

He was recently put on suicide watch for two days, in which he was stripped to his underpants, against the advice of prison psychiatrists. He remains on a regulation that keeps him alone in his cell 23 hours a day and requires him to be checked every five minutes, and he is shackled hand and foot when he has visitors.

In December the UN began an investigation into his treatment to see if it amounted to torture. The Foreign Office said that it was unable to release any information on an individual's nationality without that person's consent.

In general terms, the government normally will not intervene in cases of dual nationality where the person is held in the other country, but there are exceptions on humanitarian grounds, including claims of inhumane treatment.

• This article was amended on 2 February 2011 to correct a misquote from Alison Harvey of the Immigration Law Practitioners' Association