UPDATE:

NEWARK--More than 4,000 longshoremen unexpectedly walked off the job this morning, shutting down the port terminals both in New Jersey and New York, causing truck traffic to immediately start backing up throughout Port Newark and Port Elizabeth.

Port officials were taken by surprise by the labor action, and were unsure what precipitated it.

"We do know it's an illegal walk-off," said Phoebe Sorial, general counsel for the Waterfront Commission of New York Harbor. "We did not hear about it beforehand and we don't know why it's happening."

The New York Shipping Association, which represents the terminal operators, ocean carriers, and stevedores, was also in the dark.

"I can tell you we don't know why they walked off," said spokeswoman Beverly Fedorko. "It came as a surprise to everyone. We're trying to ascertain just what is going on."

She said the NYSA was not currently negotiating with the International Longshoremen's Association, which has been working under an existing collective bargaining agreement.

And even the ILA was uncertain about that sparked the wildcat strike. Union spokesman Jim McNamara said there is some anger by the rank-and-file over what they see as interference by the waterfront commission in their collective bargaining agreement, but added that he was also still looking into the matter.

"They see the commission as a threat to their livelihood, making life miserable for longshoremen," McNamara said. "They have gone way beyond their role of licensing to break down the gains of the ILA."

At the APM Terminal in New Jersey, a large group of dockworkers gathered outside the gates of the shipping company, but none would talk about why they had walked off the job at Port Elizabeth.

Late in the evening, the ILA put out a statement urging its members to return to work.

"We have heard your voices, we have heard your concerns, and we have taken action on your behalf," the union said in a statement. ""We urge all ILA members to return to work and will continue to report to you on the progress we make resolving all concerns of our hard working and dedicated ILA workforce."

The NYSA said it saw the statement, but declined to comment on it.

The port was effectively shot down after Port Authority of New York and New Jersey moved to stop incoming truck traffic, although police reported that local access roads throughout the morning were a standstill, with thousands of trucks idled. Officials said no ships were being unloaded.

"As the agency that oversees the largest port complex on the East Coast, we strongly urge the ILA members to return to work immediately and resolve their differences after they return," said the Port Authority in a statement. "In the meantime, Port Authority Police are actively working to ensure public safety for all of the stakeholders at the port."

The busiest port on the eastern seaboard, the combined terminals of New York Harbor at Port Newark, the Elizabeth-Port Authority Marine Terminal, the Howland Hook Marine Terminal, the Brooklyn-Port Authority Marine Terminal, the Red Hook Container Terminal, and the Port Jersey Port Authority Marine Terminal, handle more than 3.3 million cargo containers a year, moving $200 billion in goods.

Ted Sherman may be reached at tsherman@njadvancemedia.com. Follow him on Twitter @TedShermanSL. Find NJ.com on Facebook.