BUENOS AIRES, May 14 (Reuters) - Striking Argentine farmers on Wednesday pressed an offer to suspend their protest over an export tax hike if the government agrees to negotiate and modify the measure.

Farmers in Argentina, a leading exporter of soy, wheat and corn, are locked in a two-month standoff with the government over a new sliding-scale export tax that farmers say effectively caps prices for their goods.

They went on strike last week, halting grains sales and disrupting shipments. The protest has raised supply fears on global markets since Argentina accounts for nearly 3 percent of world food exports.

It is the the second strike since mid-March and came after negotiations with aides to President Cristina Fernandez collapsed last week.

"We're waiting for the government to take a step forward with a solution on the export taxes, and based on that, obviously we would lift the strike," said Luciano Miguens, president of the Argentine Rural Society.

Government officials have called on farmers to first lift the strike to clear the way for possible talks.

On the strike's sixth day, farmers parked tractors along highways and gathered in roadside protests, a day after a farm leader offered to lift the protest for 24 hours if the government offers to negotiate.

The strike was originally planned to last until Thursday, but farm leaders say it could be extended it if there is no agreement.

President Fernandez has rejected calls for a change in the tax scheme and defended it as a way to redistribute wealth and contain inflation.

She has not spoken publicly about the strike since it began on May 8, but local media say Fernandez will likely address the issue during a speech later on Wednesday.

The strike is proving a stiff political test for Fernandez, who took office just five months ago and has seen her government's popularity ratings slide as some Argentines complain about her handling of the standoff.

March's three-week strike forced some exporters to renege on their contracts, while soy-processing plants were forced to shut as their stocks of soybeans ran out.