New York and New Jersey's cargo terminals shut down on Friday after more than a thousand longshoremen walked off the job, shuttering one of the country's busiest port networks.

The employees stopped working around 11 a.m. It was not immediately clear what had prompted the walkout.

"To run a picket you need a permit and there wasn't one issued by the Port Authority," said one Port Authority official who requested anonymity.

The walkout came as a surprise because "there were no major issues that we knew of to precipitate this," the official said, adding that more than a thousand people walked out.

The Port Authority of New York and New Jersey, which owns the terminals, issued a statement urging members of the International Longshoremen's Association to "return to work immediately and resolve their differences after they return."

Over $200 billion worth of cargo moved through the port in 2014, according to the agency. Approximately a quarter of U.S. gross domestic product is accounted for in an area that is within a 200 to 250 mile radius of the ports.

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Port Authority police were dispatched to the terminals to ensure public safety, according to the statement.

Beverly Fedorko, a spokeswoman for the New York Shipping Association, which represents the terminal operators, said the longshoremen had not informed management of the "illegal" walkout. "We don't even know why," she said.

An emergency meeting between the union and management was scheduled for 3 p.m., Fedorko said.

A spokesman for the union did not immediately respond to a request for comment on Friday.

Text messages from the Port Authority's mobile alert system warned at 10:42 a.m. EST that there was "potential for heavy volume on all port roads." This was followed at 11:20 a.m. EST by a message that as a result of the work stoppage "no new trucks would be allowed to queue on the port roadways. Do not send trucks to the port."

The walkout affects several terminals, including Port Newark and terminals in Elizabeth, New Jersey; Bayonne, New Jersey; and the New York City borough of Staten Island.

The port system is the third busiest in the United States and has 3,500 registered longshoremen, Fedorko said, although the number of workers on duty per day fluctuates depending on ships and other factors.

The terminals annually handle nearly six million 20-foot equivalent units, or TEUs, according to the Port Authority. A standard 40-foot container equals two TEUs.