Updated at 4:16 a.m. ET: LOS ANGELES -- An agreement was reached Tuesday night to end the crippling strike at the ports of Los Angeles and Long Beach, according to Los Angeles Mayor Antonio Villaraigosa.

The announcement came just hours after federal mediators arrived at the port of Los Angeles, called on by Villaraigosa to help resolve a strike that has idled most of the docks at the ports for more than a week.

The eight-day labor clash cost Southern California an estimated $8 billion, including lost wages and the value of cargo rerouted to other ports over the past week, according to Reuters.

Los Angeles and Long Beach together account for nearly 40 percent of all U.S. container imports.

George Cohen, director of the Federal Mediation and Conciliation Service, and Scot Beckencaugh, deputy director for mediation services, arrived Tuesday night to begin talks between shipping and union officials, the mayor's office announced.

It was unclear whether a vote had been planned prior to their arrival or what role they played in the developments late Tuesday.

The mayor flew back from a trip abroad to help bring an end to the work stoppage, staged by clerical workers who use computers to help track the progress of shipments into and out of the nation's busiest port complex.

Villaraigosa, a onetime labor activist who had been in Latin America pitching Southern California's port operations to manufacturers, shippers and retailers there, arrived at the harbor at about 11 p.m. Monday, joining the negotiations in the hope of brokering a deal.

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Tuesday morning, he said, he called to request help from a federal mediator. The mayor said he also discussed the matter with California's two Senators, Dianne Feinstein and Barbara Boxer, and that he has placed a call to the White House.

Despite claims by both sides that they had made significant concessions in the talks so far, Villaraigosa said at a news conference that neither had moved on issues of top concern to the other.

In particular, he said, the union, which is worried about outsourcing jobs, might need to compromise on other issues to get movement on its top priority.

As the talks dragged on, the clerical workers continued to walk picket lines.

Each side blamed the other for the slow pace of negotiations.

John Fageaux, spokesman for Local 63 of the International Longshore and Warehouse Union, said his organization had backed down on a demand that the companies re-hire 51 positions that the union said had been outsourced, but received no productive response from the employers.

But Stephen Berry, an attorney representing the shipping companies, said those jobs had never been outsourced in the first place. Instead, he said, they represented positions that had been held by clerical workers who were not replaced after they retired.

For his part, Berry said that the shipping companies had agreed to one of the union’s key demands, saying that they would hire certain temporary workers from the union’s hiring hall, rather than going to outside contractors.

But he said that the union was not satisfied with that offer. He said the union failed to recognize that the economy had still not recovered from the boom years.

The stoppage at 10 of the port's 14 terminals will not affect holiday shipments, experts said, because the toys, books, electronics and clothes aimed at the gift market arrived months ago.

But Villaraigosa said it affects about 20,000 truck drivers, retailers and others who are awaiting shipments for upcoming seasons.

The ports of Los Angeles and Long Beach together handled more than $400 billion in goods arriving or leaving the West Coast by ship last year, according to Reuters. Experts say the ports directly or indirectly support 1.2 million Southern California jobs - workers involved in moving freight to or from the shipping complex. 

NBCLosAngeles.com's Sharon Bernstein and Annette Arreola and Reuters contributed to this report.