U.S. East Coast and Gulf Coast dockworkers began their first large-scale strike in nearly 50 years on Tuesday, halting the flow of about half the country's ocean shipping, after negotiations for a new labor contract broke down over wages.

The strike blocks everything from food to car shipments across dozens of ports from Maine to Texas, in a disruption analysts warned will cost the economy billions of US dollars a day, threaten jobs and potentially stoke inflation.

The International Longshoremen's Association union, which represents 45,000 port workers, had been negotiating with the United States Maritime Alliance (USMX) employer group for a new six-year contract ahead of a midnight Monday deadline.

The ILA said in a statement it shut down all ports from Maine to Texas at midnight local time after rejecting USMX's final proposal, adding the offer fell "far short of the demands of its members to ratify a new contract".

The ILA's leader, Harold Daggett, has said employers such as container ship operator Maersk and its APM Terminals North America have not offered appropriate pay increases or agreed to demands to stop port automation projects that threaten jobs.

"We are prepared to fight as long as necessary, to stay out on strike for whatever period of time it takes, to get the wages and protections against automation our ILA members deserve," Daggett said on Tuesday.

The USMX said in a statement on Monday it had offered to hike wages by nearly 50 percent, up from a prior proposal. Daggett, meanwhile, said the union is pushing for more, including a US$5 per hour raise for each year of the new six-year contract.

USMX did not immediately respond to requests for comment.

Hundreds of dockworkers demonstrated on Tuesday at a New York City area shipping terminal in Elizabeth, New Jersey, carrying signs and shouting slogans as music blared and vendors hawked food. Daggett arrived to rally them with cheers of "ILA all the way!" (Reuters)