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 United States' busiest port networks.Employees stopped working around 11 a.m. (1600 GMT). The reason was unclear."To run a picket you need a permit and there wasn't one issued by the Port Authority," said a Port Authority official who requested anonymity.The work stoppage was 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 had walked out.The Port Authority of New York and New Jersey, which owns the terminals and leases them to port operators, 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. About a quarter of U.S. gross domestic product is accounted for in an area within a 200 to 250 mile radius of the ports.Port Authority police were sent to the terminals to ensure public safety, according to the statement. The Port Authority official said there had been no word of any arrests by evening.Beverly Fedorko, a spokeswoman for the New York Shipping Association (NYSA), which represents the terminal operators and ocean carriers, said the longshoremen had not informed management of the "illegal" walkout. "We don't even know why," she said."The Contract Board (of the NYSA) called an emergency meeting. Representatives of the Contract Board are members of management and of the union. The management showed up and the union did not," Fedorko told Reuters late Friday afternoon.The NYSA is weighing legal options, she said.Attempts to contact officials and a spokesman for the International Longshoremen's Association were unsuccessful.New York and New Jersey ports are a major entry point for crude oil and an exit for refined products such as gasoline and heating oil. It was unclear if energy sector workers in the ports were participating in the walkout."I have not seen or heard anything yet that the strike was affecting the gasoline or heating oil futures markets," said Dominick Chirichella, senior partner at the Energy Management Institute in New York. Prices for both commodities were higher, in line with market fundamentals.A text message from Port Authority's mobile alert system at 11:20 a.m. EST said 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 and Bayonne, New Jersey, and the New York City borough of Staten Island.Another text at 3:13 p.m. said the marine terminals had been safely cleared, and thanked operators for their cooperation.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 (TEUs) of containerized cargo, according to the Port Authority. A standard 40-foot container equals two TEUs.