BUENOS AIRES, May 12 (Reuters) - Argentine farmers, threatening to extend the strike they resumed last week, met on Monday with regional officials to seek support in their standoff with the national government over export taxes.

Farmers in Argentina, one of the world's top suppliers of staples such as corn and soybeans, last week went on strike for the second time in less than two months after negotiations with the government collapsed.

They vowed to halt sales of grains for export until May 15, threatening to disrupt shipments and raising supply fears on global markets. At roadside protests across the country at the weekend, farm leaders said the strike might last beyond Thursday.

Farm leaders' contacts with regional governors in agricultural regions offered some possibility of fresh negotiations, but talks between ruling party provincial leaders and the rebel farmers could irk President Cristina Fernandez.

"The (governors') political gesture is very important ... it's a concrete expression that this conflict has got to be resolved," said Eduardo Buzzi, president of the Argentine Agrarian Federation.

The federation, which groups mainly small-scale farmers and ranchers, is one of the four groups leading the protests, which began after the center-left administration imposed a sliding-scale export tax on grains that lifted the rate on soy and sunseeds.

CHALLENGE

Farmers' protests have handed Fernandez her biggest political test since she took over from her husband in December.

The three-week strike in March caused beef and dairy shortages -- driving up prices as her government battles inflation. It also slowed factory output as soy-processing plants were forced to close, and sparked anti-government pot-banging protests in the cities.

It also has caused jitters in financial markets, sending bonds and the peso currency lower in recent weeks. Monday's meeting between strike leaders and Cordoba Governor Juan Schiaretti, a member of the ruling Peronist party and a Fernandez ally, suggested some growing impatience within party ranks for the row to be resolved.

"He's the first governor to meet with the heads of the main farm groups, and he expressed to us that the export taxes should be lowered," Buzzi said.

Officials in the provincial governments of Buenos Aires and Santa Fe also met farmers, who maintained their roadside demonstrations. Some blocked roads to stop trucks passing, local media said.

The three regions straddle the Pampas plains that are the heart of Argentina's farming belt.

In the town of Gualeguaychu, which has become the focus of the farm strike, local farming leader Alfredo de Angeli expressed some optimism over the meetings with regional officials.

"This is a step forward," he said, adding that he planned to lobby regional officials in the coming days.

"My message is going to be 'keep banging those pots and pans' so we can wake up the deputies, the provincial governors so they start to defend people in their regions," he added, referring to the pot-banging protests that most Argentines associate with the economic and political meltdown of 2001/02.