Cristina Fernandez de Kirchner and visiting Luiz Inacio Lula of Brazil made the declaration, covering energy, transport and space satellite cooperation, Mercopress reported Sunday.
"The strategic alliance between Argentina and Brazil is of the utmost importance," said Lula.
Uranium enrichment provides the fuel needed to operate nuclear plants, but is also used for nuclear weapons, which is a leading concern in Washington.
Argentina has previously been praised for its role in opposing nuclear proliferation. Argentina has two operating nuclear plants and is seeking to bring another online by 2010, and Brazil also has two operating nuclear plants and is working on a third.