Paul Charles Wilkins, 70, of Littleport in Cambridgeshire, travelled to California in January to "engage in illicit sexual conduct" with boys aged 10 and 12, US officials claim.
Wilkins has denied four charges, the US District Attorney's Office said.
He was jailed in the UK in 2011 for possessing indecent images of children.
He admitted 16 charges and was jailed for 56 months at Cambridge Crown Court, the Crown Prosecution Service for the East of England said.
Wilkins, who holds dual UK and US citizenship, was on probation when he left the UK on 31 January.
He had set up an online site enabling him to arrange to meet boys, but was caught in a sting operation in Palm Springs, California, by undercover Homeland Security Department special agents.
He had allegedly paid one agent $250 (Â£170) to have sex with a nine-year-old Mexican boy.
In February Wilkins was charged with travelling with intent to engage in illicit sexual conduct with boys, which carries a statutory maximum sentence of 30 years, and attempted sex trafficking.
In June further charges of transporting and possessing indecent images of children were added.
Earlier this month Wilkins "was arraigned on that indictment... at which time he pleaded not guilty to the charges", a spokesman from the United States Attorney's Office for the Central District of California (Los Angeles), said.
Wilkins, described by US Attorney Eileen M Decker as a predator, was initially expected to face trial in the US on 19 July.
That trial date has now been moved to 15 November.
Wilkins was detained as part of Homeland Security's Operation Predator, an international initiative to protect children from sexual predators.
A convicted sex offender allegedly caught trying to pay US undercover officers for sex with a boy has denied a series of sexual assault charges.