LOS ANGELES -- Crucial talks between feuding U.S. shippers and longshoremen remained deadlocked Tuesday, despite calls by President George W. Bush for an end to an economically devastating shutdown of West Coast ports.

Shipping line bosses and dock workers met separately with a federal mediator as Washington sought to force a breakthrough in the closure that is costing the U.S. economy around a billion dollars a day and hitting Asian exporters.

But while both sides met with a federal official to discuss the crippling lockout entering its third day, union chiefs refused to enter into mediated talks despite growing pressure from the White House.

"There are no negotiations of any sort today," said Steve Stallone of the International Longshoremen's and Warehousemen's Union (ILWU) which represents 10,500 dock workers.

"We are not accepting federal mediation at this time, we are simply listening to what (the mediator) has to say, but there is no mediation going on, nor is there a decision to do that," he added.

But even those "informational" talks broke down amid mutual acrimony, with union officials storming out after being greeted by gun-toting security guards employed by the shipping representatives, Stallone said.

Officials of the Pacific Maritime Association, which represents shipping lines, said they needed the protection of the armed guards.

The indefinite shutdown of the 29 ports between the California City of San Diego and Seattle in the northwestern state of Washington began Sunday when shippers locked out unionized workers accusing them of staging a go-slow, AFP reported.

The two sides met Monday in San Francisco, but little progress was made.

Bush weighed in on the dispute, saying he was worried by the shutdown culminating a five-month-old constract dispute, amid growing fears the stoppage could be extended, inflicting huge damage on the struggling U.S. economy.

"We are worried about it. We are closely monitoring it," Bush said at the White House, adding that it came at a bad time for the U.S. economy.

"There is a federal mediator on the ground and I urge both parties to utilize the mediator," he said, adding that it was "important for our economy" to get the parties back to work and reopen the ports.

Scores of ships carrying hundreds of millions of dollars of goods were meanwhile queuing up off the ports, while hundreds of lorries remained blocked outside the padlocked gates as union workers picketed the docks.

Perishable goods such as meat and fruit and vegetables were in danger of going bad, while millions of dollars of toys and other goods aimed at the crucial Christmas holiday market were being held up.

Union workers have been without a contract since July 1 amid demands by employers that the union allow some jobs to be replaced by electronic tracking systems and other technology.

The PMA has rejected a call from the union to reopen contract negotiations by removing the issue of technology from the talks. "Technology cannot be taken off the table," said spokesman Tom Edwards. "It is the core issue in these talks. We must modernize our terminals and bring our waterfront into the 21st century. The union cannot turn back that clock."

The PMA called for the union to extend its last labor contract and resume normal operations, but the union has refused, although stressing its members want to return to work as soon as possible.

More than $300 billion worth of goods, or about seven percent of the U.S. gross national product, pass through West Coast ports annually, much of it consisting of trade with America's closest partners in East Asia.