Argentine shipping workers will delay British-flagged vessels in port as tensions between the two countries rise in the run-up to the 30th anniversary of the Falklands war, a union leader said on Tuesday.

Argentina's CATT transport workers' federation said its members would stall the loading and unloading of British ships to press London to open sovereignty talks with Buenos Aires over the disputed South Atlantic islands.

We're going to economically hurt British-flagged ships or British ships flying the flag of convenience, said Omar Suarez, head of the SOMU shipping workers' union that belongs to the CATT federation.

Shipping companies routinely register vessels under flags of convenience, placing their legal ownership in jurisdictions such as Panama where tax and other regulations are lenient.

We'll let them in but with a delay of six to 12 hours, Suarez told Reuters. It will go on indefinitely until Britain negotiates with our government over the (islands') sovereignty.

Britain and Argentina went to war in 1982 over the Falklands, which are called Las Malvinas in Spanish. London has refused to start talks on sovereignty with Buenos Aires unless the 3,000 islanders want them.

Argentine trade unions traditionally have strong ties with the ruling party of President Cristina Fernandez, who has intensified demands for Britain to agree to sovereignty talks in recent months.

Suarez said the port workers' protest had already begun, but officials said shipping activity remained normal.

Argentina is one of the world's top suppliers of corn, soybeans and byproducts, and most of its grains are shipped from the constellation of terminals around the city of Rosario.

Everything's normal here, said Guillermo Wade, manager of the country's Port and Maritime Chamber in Rosario.

In the southern grains port of Bahia Blanca, which is also home to an import terminal for liquefied natural gas (LNG), the British-flagged Ruby was being unloaded after its arrival on Monday, a shipping official said.

The vessel, owned by British oil company BP, was carrying 55,000 tonnes of LNG imported by the state energy company Enarsa, a company spokesman said. Suarez said the Ruby would not be affected by the protest.

Falklands-flagged ships are not allowed to dock at ports in Brazil, Argentina, Paraguay and Uruguay as part of a ban by the regional trade bloc Mercosur.

The ban, which has been condemned by Britain, does not include British-flagged civilian ships that supply the islands, although it applies to military vessels.

Long-standing diplomatic tensions over the islands have bubbled up in recent months. Argentina has been angered by oil exploration in the Falklands by British companies and Britain's decision to send one of its most sophisticated warships on patrol in the area.

(Additional reporting and writing by Luis Andres Henao; Editing by Helen Popper and Lisa Shumaker)