Russian President Vladimir Putin has signed a series of agreements on nuclear energy in Argentina.
It came during his tour of Latin America aimed at increasing Russian influence in the region.
The nuclear agreements will see the Russian atomic energy corporation Rosatom get involved in the construction of units in Argentina's nuclear Atucha 3 power plant.
Mr Putin will go to Brazil to attend the football World Cup final on Sunday.
Russia will host the next tournament in 2018.
After Sunday's match, Mr Putin will attend a summit of the Brics emerging economies - also including India, China and South Africa - in the Brazilian city of Fortaleza.
'Grateful for support'
Correspondents say Argentina is desperate for foreign investment as it faces a possible default after investors rejected its debt restructuring.
The first stop of Mr Putin's Latin America tour was Cuba - and he also made an unexpected visit to Nicaragua, the first by a Russian leader.
"Today, co-operation with Latin American states is one of the key and promising lines of Russia's foreign policy," Mr Putin told Cuban state media.
Mr Putin is looking to tighten ties with this region, partly to balance a slump in relations with the West over Russia's intervention in Ukraine, the BBC's Sarah Rainsford in Havana reports.
In Cuba he met Fidel Castro, who led Cuba for nearly half a century before handing over power to his brother, Raul, in 2006 because of ill health.
Russian media said they discussed international relations and Russian-Cuban ties.