A mentally ill British man has pleaded guilty to US charges he plotted to set up an al Qaida training camp on a ranch in a remote part of Oregon.

Haroon Aswat admitted he travelled to Bly, Oregon, in 1999 at the direction of Mustafa Kamel Mustafa, a double-amputee and radical cleric based in London and also known as Abu Hamza.

His orders were to help train recruits "who wanted to participate in jihad on behalf of a terrorist organisation," he said in a barely audible voice at the sentencing in court in New York City.

Before pleading guilty to conspiracy and supporting terrorism, Aswat, who was born in Sheffield, told US District Judge Katherin Forrest that he had been diagnosed with schizophrenia at 20.

The judge ruled that the sentencing could go forward because the defendant was under medical care and showing no symptoms.

Aswat, 40, was charged in 2005 in the plot to establish the camp for military training at a site known as Dog Cry Ranch. Prosecutors accused the defendant of arriving in Oregon with instructions on how to make bombs and poisons.

According to court papers, one communication between the conspirators said that the property was located in a "pro-militia and firearms state" that "looks just like Afghanistan" and that the group was "stockpiling weapons and ammunition".

But the camp never materialised beyond a dozen people taking target practice, authorities said.

Aswat faces up to 20 years in prison at his sentencing on July 31. It was unclear whether he could get credit for the nearly 10 years he spent behind bars in the UK fighting extradition to the US.

Mustafa was sentenced this year to life in prison following a trial in the same Manhattan courthouse.

Authorities say Mustafa turned London's Finsbury Park Mosque in the 1990s into a training ground for Islamic extremists, attracting men including September 11 conspirator Zacarias Moussaoui and shoe bomber Richard Reid. He claims to have lost his hands fighting the Soviets in Afghanistan.