Buenos Aires  - Truck drivers have joined farmers protesting on Argentine roads, sharply increasing the risk of supply problems in urban areas.

On Thursday, truck drivers continued to protest the damage that the farmers' strike launched in March has caused to their own work.

The drivers' move gave a further twist to the long-standing conflict between farmers and the Argentine government led by President Cristina Fernandez de Kirchner, by putting at risk supply lines to urban areas impacting the work of industry and other businesses.

Supermarket chains and food companies warned that their stock of fresh goods, dairy products and national staple beef would run out within a few days, while long-distance bus services were limited in several regions due to road blockades.

The crisis in Argentina's powerful farming sector has been brewing since March, when an increase in export tariffs for soybeans and sunflower went into effect, tying tariffs to international market prices.

The average tariff was increased from 35 to 41 per cent and applied to almost all of the surplus if the price for soybeans were to rise above 600 dollars a tonne, although the government has since modified this to set a tax ceiling.

The positions of the government and agricultural producers have remained far apart.

In March, a complete blockade of agricultural production caused severe food shortages in large urban areas and provoked traffic problems. Earlier this month, farmers opted to block the export of cereal and other key products, like soybeans.

In their third protest, farmers again took to the roads last week. Although their leaders asked that they not disturb traffic, the passage of trucks carrying agricultural produce was blocked in some areas.

In a strike that was set to end Monday but was prolonged until the end of the week, farmers were blocking the sale of cereal and oil- based crops for export. They however allowed the sale of cattle for meat from Tuesday, to prevent shortages in the internal market.

Cereal transport trucks have since Monday blockaded roads in the central provinces of Buenos Aires, Santa Fe, Cordoba and Entre Rios - in the so-called Wet Pampa - amid great tension with farmers protesting on the side of roads. Several towns are virtually isolated as a result of the moves.

Drivers were protesting the lack of work in recent months. However, many observers interpreted their move as a government- sponsored measure. The truck-drivers' union is led by Pablo Moyano, the son of the pro-Kirchner General Workers' Union (CGT) Hugo Moyano.

Pablo Moyano stressed that drivers are "hostages" in the conflict and that farmers' associations will be to blame for any supply problems.

Interior Minister Florencio Randazzo stressed that "it is not legitimate that people blockade roads, be it truck drivers or the farming sector." However, he noted that "it is legitimate that truck drivers protest over their right to work."

"There are producers, truck drivers and small and medium enterprises that want to work, that have to do it and cannot do it because of the farming lockout," the minister said.

After years of meagre earnings, Argentina's farmers don't want the export duties to cut into their profits just as they have begun to see more money flowing in with higher world food prices.

The halt to agricultural exports has proved expensive for the government because a large part of its income comes from export duties. (dpa)