A threatened strike by dock workers that would have closed much of the Port of Boston and other East Coast ports has been put off for at least 30 days, after shippers and longshoremen came to terms on a major sticking point in their contract talks.

A federal mediator overseeing the negotiations announced Friday that the US Maritime Alliance, an association of shipping companies, and the International Longshoremen’s Association had reach an agreement in principle on the main issue: additional payments for handling container cargo.