AFP - The United States has pressed Japan to throw open its lucrative markets to foreign farm products, as US lawmakers demanded an end to curbs on American beef.

US Agriculture Secretary Tom Vilsack urged Tokyo to ease restrictions on agricultural imports ahead of an April visit to the country, which he said would also deal with a long-standing beef spat.

"My mission on this trip will be to continue to push hard to open markets," Vilsack said, announcing the April 5-9 visit.

According to agriculture department's figures, Japan is the third largest market for US exports, with sales last year of 11 billion dollars.

But the two country's have frequently sparred over measures protecting sensitive domestic agricultural markets.

Japan banned US beef in December 2003 after the brain-wasting cattle disease bovine spongiform encephalopathy (BSE) was found in a US herd.

Japan had until then been the US cattle industry's biggest export market.

The ban nearly grew into a full-blown trade war, with US farm-state senators pressing for sanctions unless Tokyo opened up its markets by the end of 2005.

Japan agreed in 2006 to resume US imports on the condition that age and portion limits were imposed on cattle at the time of slaughter.

In tandem with Vilsack's push, top US lawmakers on Tuesday demanded the "scientifically unfounded" restrictions on US beef be eliminated.

In a letter to Japan's ambassador in Washington, Senate Finance Committee chairman Max Baucus, a Democrat, and the panel's top Republican, Charles Grassley, said the barriers were an obstacle to better trade ties.

"Millions of Americans consume US beef from cattle of all ages every day, so the safety of this product cannot seriously be in doubt," the senators wrote.

"This scientifically unfounded barrier to imports of US beef is causing economic hardship for cattle and beef producers in Montana and Iowa," two major farm states.

Vilsack told reporters that he was hopeful progress could be made on ending the beef spat.

"I am looking forward to an opportunity to discuss how that market, which has been closed for a considerable period of time, can be reopened."

"My hope is that we can make progress. There's no guarantee," he said adding that efforts to reopen the market were "complicated" by the previous George W. Bush administration's policy of seeking similar deals with all nations on beef.

Meanwhile senators Baucus and Grassley also called for an end to "preferential treatment" of Japan Post in the country's insurance, banking, and express delivery markets at the expense of private competitors.

The US calls for better treatment come amid a major push by President Barack Obama to double US exports, in the hope of clawing back some of the millions of jobs lost in the economic crisis.

A US agriculture official said Vilsack's focus would also be on increasing exports to Japan of grains and fruits, key US exports.