The New York Shipping Association says the stoppage by International Longshoremen's Association members is related to a protest over the move of some work done by ILA workers in Philadelphia to a non-ILA facility in New Jersey.
A shipping association spokeswoman says the stoppage is affecting 12 ships waiting to be unloaded in the ports of New York and New Jersey. She says passenger terminals on New York's west side are not affected.
The Port Authority of New York and New Jersey says it is taking steps to minimize the impact of the stoppage.
An ILA spokesman didn't immediately return a message Tuesday.