LOS ANGELES (AFP) - A court jailed a Somali-US man for 30 years on Wednesday for plotting to blow up a crowded Christmas tree ceremony attended by thousands in the US state of Oregon.

Mohamed Mohamud, 23, was found guilty last year following a three-week trial over the 2010 plot, after he was snared in an FBI sting operation involving a fake bomb that he tried to detonate.

Prosecutors said that Mohamud actively participated in the plot that could have killed thousands attending the Christmas tree lighting ceremony on November 26, 2010, had the bomb been real. But lawyers for Mohamud claimed he was tricked by undercover agents from the Federal Bureau of Investigation.

In June that year an FBI undercover agent contacted Mohamud, a naturalised US citizen and former student at Oregon State University. After meeting the agent, Mohamud said he had been thinking of conducting a "holy war" since the age of 15, and suggested the plot to bomb the traditional pre-Christmas ceremony in the Oregon capital of Portland.

FBI undercover operatives cautioned Mohamud several times about the seriousness of the plan, noting that there would be many children at the event. But Mohamud responded that he was looking for a "huge mass that will... be attacked in their own element with their families celebrating the holidays", according to court documents.

Mohamud also collaborated with the undercover agents in detonating a device in a rehearsal in a remote location, and recorded a video in which he read a written statement that offered a rationale for his bomb attack. He was arrested shortly after trying to detonate the fake bomb as the Christmas tree ceremony got under way in Portland's Pioneer Courthouse Square.

Mohamud was convicted in January last year of attempting to use a weapon of mass destruction. He apologised in court on Wednesday shortly before being sentenced by US District court Judge Garr M. King, whose assistant Pamela Graham confirmed the 30-year jail term.

"The things I said and did were terrible... I want to apologise to everyone, to the community," he said, according to the Oregonian newspaper's website, which said he also said sorry to the Muslim community.

His mother expressed disappointment at the sentence. "We are Muslim, and our religion does not (allow us) to kill innocent people," she told reporters, the Oregonian reported.