New Jersey Governor Chris Christie was told about politically motivated traffic jams near the George Washington Bridge while they were crippling a town whose mayor refused to back his re-election, a US prosecutor said for the first time since the scandal became public.

The assertion came on the first day of the trial of Bridget Anne Kelly and Bill Baroni, who are accused of deliberately closing access lanes to the bridge to paralyse traffic in Fort Lee, New Jersey, over several mornings during the first week of school in September 2013.

Prosecutors say the goal was to retaliate against Mayor Mark Sokolich, a Democrat who refused to cross party lines to back Christie, a Republican.

A former Port Authority official, David Wildstein, pleaded guilty and is expected to testify that he conspired with Kelly and Baroni to punish Sokolich and cover up their crimes until the scheme came to light weeks later.

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In his opening statement, a prosecutor said today that Wildstein and Baroni spoke to the Governor on the third day of the traffic jams during a September 11 memorial at the World Trade Centre in New York.

"During those precious few minutes, they bragged about the fact that there were traffic problems in Fort Lee and that Mayor Sokolich was not getting his calls returned," Assistant US Attorney Vikas Khanna told jurors in federal court in Newark, New Jersey.

Christie has denied he knew anything of the traffic plot until weeks after it happened.

Spokesman Jeremy Rosen declined to comment today, citing the Governor's earlier statements that he didn't know about the lane closings at the time. Wildstein's lawyer had previously said "evidence exists" that Christie knew about the lane closings when they occurred.

While Christie's political opponents have investigated the closings, and he commissioned a probe that cleared him of wrongdoing, this marked the first time a prosecutor publicly endorsed Wildstein's view that Christie knew of the plot as it was happening.

Kelly, Christie's former deputy chief of staff, and Baroni, the former deputy executive director at the Port Authority of New York & New Jersey, have pleaded not guilty and promise a defence that will shed light on the inner workings of Christie's Administration.

In their opening statements, lawyers for Baroni and Kelly derided Wildstein, 54, as a serial liar, a lifelong manipulator and a political operative who was Christie's enforcer to help him in his quest for the White House. They said Wildstein will lie to the jury about Kelly and Baroni to try to please prosecutors and avoid prison when he's sentenced.

"David Wildstein is a vicious guy" who was Christie's "fixer," Michael Baldassare, Baroni's lawyer, told the jury. "He's a bully," Baldassare said before resorting to profanities to describe him.

As recently as yesterday, Christie said he didn't know anything about the lane closures until later and would be willing to testify at the trial if called to do so.

"I would have no problem if called to testify by either side," Christie, now a top campaign adviser to Republican presidential nominee Donald Trump, said on CNN. "But the fact is that I won't because I really don't have any knowledge of this incident at all."

- Bloomberg