Donald Trump campaign loyalist and New Jersey Governor Chris Christie was fully aware that his top aides shut down highway access lanes to a Manhattan bridge to cause a massive traffic jam as retaliation against one of Christie's political foes, federal prosecutors said in court.

The closures, which tied up thousands of New Jersey and New York commuters heading to the nation's busiest bridge for hours over several days, were intended to punish a local mayor for refusing to support Christie in his re-election bid, said prosecutors.

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Top aides had "bragged" to the governor about the lane closings at a memorial service for those killed in the 11 September terrorist attacks, saying that they had been shut to "mess" with Democrat Mayor Mark Sokolich of Fort Lee, New Jersey.

Sokolich's desperate entreaties to open the lanes for "public safety" fell on deaf ears. Aides later claimed the lanes had been shut as part of a "traffic study."

Assistant US Attorney Vikas Khanna said former Christie confidante and Port Authority executive David Wildstein will testify that he told Christie about the plan to close lanes on the George Washington Bridge which connects upper Manhattan to Sokolich's town of Fort Lee, CNN reported. The Port Authority is in charge of the region's bridges.

When news of the deliberate lane closings first emerged, it was widely seen as the death knell of Christie's political career. Critics didn't find it credible that aides would engineer such a retaliatory, devastating traffic jam without instructions from their boss.

But Christie has found a second wind as a frequent outspoken supporter and a regular figure in the Trump campaign.

If Trump wins the race for the White House, Christie, now chairing the candidate's transition effort, will likely get a plum federal position. But the latest revelations could have blowback in the Trump campaign.

The courtroom accusations were made at the start of the trial of two former Christie administration officials — Bill Baroni and Bridget Anne Kelly — who are charged with closing the lanes in 2013 and then covering it up. They have both pleaded not guilty to conspiracy and fraud in connection to the lane closures, which is known as "Bridgegate."

Kelly sent an email in August 2013 saying, "Time for some traffic problems in Fort Lee" after confirming that Sokolich would not endorse Christie, according to prosecutors. A month later, two of three access lanes to the George Washington Bridge were shut down. Baroni, then the highest-ranking official at the Port Authority of New York and New Jersey, ignored Sokolich's repeated entreaties to open the lanes.

Defence attorneys for the officials have long claimed that Christie was well aware of the strategy and that his aides were offered up as sacrifices to pay the political price.

But the opening statement was the first time that federal prosecutors have agreed that Christie knew about the plan.

When asked if the new accusations mean that Christie himself will be charged, Department of Justice spokesman Matthew Reilly told CNN: "We have not filed any charges and we never discuss whether we will/won't file charges."

In response to reporters' queries, Christie aide, Brian Murray, pointed to comments by the governor in 2014 that he had no idea the lanes had been shut down, the New York Times reported.

In late 2015, when Christie was still running against Trump for the Republican presidential nomination, Trump told an audience in South Carolina that Christie "totally knew about it," meaning Bridgegate

Christie has recently stepped up to claim that Trump did not begin and stoke the so-called "birther" rumours that President Barack Obama was not born in the US.