Longshoremen walked off the job at ports in New York and New Jersey on Friday, putting a halt to the unloading of ships at the nation's third-busiest port.

The walkout began at about 11 a.m. It wasn't immediately clear what prompted the action. Several thousand longshoremen are employed in the ports system.

The Port Authority of New York and New Jersey, the ports' operator, said more than 1,000 longshoremen participated in the walkout.

A spokesman for the International Longshoremen's Association said the walkout took many union officials by surprise. James McNamara told NJ.com that the union has objected to what it considers interference by the Waterfront Commission of New York Harbor in its collective bargaining agreement.

No ships were being unloaded and no trucks were being allowed to enter the port during the job action. On Wednesday, the ports had reopened after being closed for two days for snow removal from last weekend's blizzard.

The Port Authority was working to get trucks that were in the port at the time of the strike out of the area. It sent an email saying, "We strongly urge the ILA members to return to work immediately and resolve their differences after they return."

The New York-New Jersey port handles more than $200 billion worth of cargo per year and is the busiest porton the East Coast and the third-busiest in the country, behind Los Angeles and Long Beach, California.