US Secretary of Labour Thomas Perez ratcheted up pressure for a resolution over a disputed new contract for dockworkers at West Coast seaports on Thursday.

Mr Perez told bosses and unions that if they didn’t reach an agreement by tomorrow, they would have to negotiate in Washington.

He issued the deadline to leaders of the dockworkers’ union and a maritime association representing the employers.

Should the two sides not succeed, Mr Perez said he will haul their leaders to the nation’s capital next week, according to Oakland Mayor Libby Schaaf.

Shifting the stage to “the shadow of the White House will place immense pressure on these parties to resolve an issue of national importance,” Ms Schaaf said.

The talks in San Francisco began nine months ago, but broke down in recent weeks. Mr Perez began overseeing negotiations on Tuesday.

Meanwhile, billions of dollars of cargo sits on ships anchored outside the 29 ports.

They cannot dock because of cargo bottlenecks at the ports.

Cargo already was moving slowly due to systemic problems. But, starting in the autumn, problems reached crisis levels due both to dockworkers slowing their work rate to comply with safety rules, and lockouts by bosses.

Dockers’ union ILWU said recently that the two sides had been “extremely close” to a deal before employers shut down ports for two days at the start of February.