James Ujaama, 48, who has served jail time for a related conviction, yesterday told the Manhattan federal court he was once "very close" to Abu Hamza, working for him at his Finsbury Park Mosque in north London in 2000 and 2001.
"Sheikh Abu Hamza ordered me to take Feroz Abbasi," he said, referring to a British man arrested by US troops in Afghanistan in 2001 and later held at Guantanamo Bay.
He is charged over the 1998 kidnapping in Yemen of 16 Western tourists, four of whom were killed, and conspiracy to set up an Al-Qaeda-style training camp in Oregon in late 1999.
Ujaama said Abu Hamza sent two men from London to help with jihad training as part of the conspiracy to open the Oregon camp.
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Ujaama, who lived in Seattle at the time, pleaded guilty in 2007 in New York to conspiring to set up the facility.
Ujaama said that he communicated with Abu Hamza when the cleric was in London and he in Seattle by telephone, fax and email.
When the Oregon plot fizzled out, the Ugandan-born Ujaama moved to London, a year after meeting Abu Hamza for the first time.
He became administrator and designer of the website of Abu Hamza's "supporters of Sharia," or SOS organisation.
He said the two became "very close" but that Abu Hamza "gave the directions and had the final say on all matters."
When Abu Hamza ordered him to accompany Abbasi to Afghanistan, Abbasi was aged 20. Abbasi gave his address as the Finsbury Park mosque on his passport and left via Karachi on November 10, 2000.
The court was shown his airline ticket.
Mustafa Kamel Mustafa, 56, better known in Britain as Abu Hamza al-Masri, has pleaded not guilty to all 11 charges.
He faces the rest of his life in a maximum security US prison if convicted in the Manhattan federal court after a trial expected to last well into May.