BUENOS AIRES, April 24 (Reuters) - A union of Argentine transporters started an indefinite strike on Monday, threatening to hit shipments at the Rosario agro-port hub by forcing exporters to rely on dwindling grain reserves at the terminals, according to local bodies.

In the province of Santa Fe, where the Rosario terminals are located, transporters prevented the passage of trucks carrying grains, practically paralyzing the entry of vehicles to one of the largest export centers in the world.

In Argentina, more than 80% of grain transportation is done with cargo trucks.

The corn and soybean harvest, which has been hit by an intense drought, has just begun, meaning traffic is usually intense at this time of year in Argentina.

The Single Freight Carriers Union (Siunfletra), which brings together truck owners, is asking for an increase in the tariffs on the transportation of merchandise and for a seat at the table at the meeting where these rates are decided.

"The transport of land cargo is in an emergency. The rates are derisory. You can't cover the costs of the truck, you don't have profitability," said Daniel Seanopolo, a representative of the port branch of Siunfletra.

A source from the chamber of grain exporters and processors (CIARA-CEC) said that shipments remained steady on Monday, although it warned that grain and by-product reserves are low due to the worst drought in decades.

Guillermo Wade, manager of the Chamber of Port and Maritime Activities (CAPyM), said "the arrival (of trucks with grains) is being affected, but there are stocks (reserves) to work with."

Seanopolo threatened that the strike will continue until they are addressed by the government.

However, a source from the Ministry of Transport told Reuters that Siunfletra is not part of any of Argentina's cargo chambers, so cannot participate in upcoming weekly meetings. (Reporting by Maximilian Heath; Writing by Valentine Hilaire Editing by Marguerita Choy)