The shutdown, the first strike by the International Longshoremen's Association (ILA) in almost 50 years, affects 36 ports from Maine to Texas, which handle an array of goods from food to electronics.

About 45,000 workers are on strike, according to the ILA.

After weeks of stalled talks, the United States Maritime Alliance (USMX), which represents shipping companies and terminal operators, had late Monday expressed some hope of a deal. But there was no agreement before the midnight deadline.

In Elizabeth, New Jersey, trucks passing by honked their horns in support of about 200 striking workers carrying American flags and signs blasting port automation as a job killer.

"Profits over people is unacceptable," one sign read.

A possible stoppage had been telegraphed for months, with the odds rising in recent weeks as the September 30 contract deadline loomed.

Analysts caution that a lengthy strike could pose a major headwind to the US economy, leading to shortages of some items and lifting costs at a time when inflation has been moderating.

The White House said President Joe Biden and Vice President Kamala Harris were "closely monitoring" the strike, with both briefed on government assessments that "impacts on consumers are expected to be limited at this time," according to a statement.

Biden was briefed late Tuesday on the situation and again called for a "strong and fair offer" to the longshoremen, the White House said, singling out "foreign-owned ocean carriers" represented by USMX.

"These foreign companies have seen record profits... and the president believes it is time they present an offer that reflects ILA workers' invaluable contribution to their success," it said.

Under the Taft-Hartley Act, Biden has the authority to order the parties to resume talks for an 80-day "cooling off" period, with union members going back to work during that time.

But Biden has ruled out such a move, citing respect for collective bargaining rights.

The National Retail Federation called on Biden to "immediately" restore operations, including by invoking Taft-Hartley, saying the strike "will have devastating consequences for American workers, their families and local communities."

And former president Donald Trump, who is seeking to take back the Oval Office, blamed Biden for the crisis, saying in Milwaukee: "He should have worked out a deal."

 Automation anxiety

The first ILA walkout since 1977 follows recent high-profile strikes at US automakers, Boeing and other employers.

The union is pressing for protections against automation-related job loss and for hefty wage hikes after dockworkers kept providing essential services throughout the Covid-19 pandemic.

Media reports say the ILA is asking for a 77 percent wage increase over six years.

USMX on Tuesday defended its latest offer to increase wages by "nearly 50 percent."

"We have demonstrated a commitment to doing our part to end the completely avoidable ILA strike," USMX said. "We look forward to hearing from the Union about how we can return to the table and actually bargain, which is the only way to reach a resolution."

The ILA responded late Tuesday, accusing USMX of attempting to "distort the facts and mislead the public" and offering details about the working conditions of its members.

"Our members feel underappreciated, especially given the sacrifices they made during the pandemic, keeping ports open and the economy moving," it said.

'We moved the world'

Oxford Economics estimated that the strike would dent US gross domestic product by $4.5 billion to $7.5 billion per week. The overall economic hit depends on the length of the strike, analysts say.

Jonita Carter, a dockworker for 23 years, said workers are feeling financially pinched by inflation and anxious about automation.

"We worked during Covid. We never stopped. We moved the world," she told AFP.

Capital Economics said fears about the economic impact of the strike were "overdone," in part because recent shocks to the supply chain have made businesses more aware of the need to bake in precautionary measures.

But Biden would have "little choice" but to take action if the situation worsens, according to the note, "forcing workers to return while negotiations continue."

"There is little chance that the administration would risk jeopardizing its recent economic successes just five weeks before a tightly contested election."

© Agence France-Presse By John BIERS