* All ports affected by dock workers' strike - chamber
* At least 2 cargo ships in main grains hub Rosario halted
* Strike started due to problem at port of Bahia Blanca
* Stevedores say to stage work stoppage next week (Adds strike indefinite, quote from union leader)
By Maximilian Heath and Hugh Bronstein
BUENOS AIRES, March 1 (Reuters) - Argentine dock workers went on strike for better conditions on Thursday, slowing the country's ports just ahead of corn and soybean harvests key to world grains markets and the local economy.
"At this time we are not mooring ships, and this will continue until we reach a deal with the port authorities," Omar Suarez, spokesman for the SOMU harbor workers' union, told Reuters.
Argentina supplies nearly half the world's soymeal, used for animal feed, as well as soyoil, used for cooking and in the booming international biofuels sector. The South American country also provides about 12 percent of soybean exports and is the world's No. 2 corn exporter after the United States.
The strike halted at least two cargo ships in the country's main grains hub Rosario, said Argentina's port chamber, known by its Spanish acronym CAPYM.
The chamber issued a statement saying all the country's ports were being affected by the work stoppage, which was called as Argentine farmers get set to bring in their crops.
"It is not possible to dock or clear ships to set sail," the statement said.
"This all started due to a problem in the port of Bahia Blanca and now the unions are extending their operations," it said. "At this point the strike is affecting the whole system."
The country's SUPA stevedores' union said it also plans to strike on March 7 to demand job guarantees. That work stoppage would affect cruise ships as well as cargo haulers.
Labor unrest is common in Argentina, where unions strive for pay raises in line with annual inflation estimated by private economists at over 20 percent.
As the global population grows to an estimated 9 billion by 2050, demand for food will nearly double, according to the United Nations. Argentina, with its vast and fertile Pampas plains, will be key to meeting that demand.
One grains-exporting company had its Rosario dock free and ready to tie up two cargo ships for loading, said Guillermo Wade, CAPYM director.
"But neither of the ships can be docked and no one knows for how long," Wade told Reuters by telephone.
The SOMU harbor workers' union is part of the labor federation that last month vowed to delay British-flagged vessels in port as tensions between the two countries rise in the run-up to the 30th anniversary of the Falklands conflict.
Britain and Argentina went to war in 1982 over the Falklands, called Las Malvinas in Spanish. London has refused to start talks on sovereignty with Buenos Aires unless the 3,000 islanders call for them, which they show no sign of doing.
Many Argentine unions have strong ties with the party of President Cristina Fernandez, who has intensified demands for Britain to agree to sovereignty talks in recent months. Oil exploration off the islands by British companies has raised the stakes in the territorial spat.. (Editing by Marguerita Choy, Bob Burgdorfer and Dale Hudson)