Under the contract, announced last week, Bolivia will eventually provide its neighbor with 36 million cubic yards of natural gas daily -- nearly four times the current amount.

Both leftist leaders -- Nestor Kirchner of Argentina and Evo Morales of Bolivia -- portrayed the deal as part of a broader drive toward Latin American solidarity.

Argentina is counting on Bolivian gas to help it overcome domestic shortages, but Bolivia's ability to meet new gas commitments has come into question since Morales announced plans to nationalize the country's hydrocarbons industry on May 1.

The decree has frightened off private investors, creating doubt Bolivia will have the infrastructure needed to increase its production. 

Last month, Bolivia's state energy company said it can fulfill current natural gas contracts to Brazil and Argentina, but will need $800 million in foreign investment over three years to meet new demand for the fuel.

Kirchner vowed that his country would help Bolivia meet its gas commitment if private industry doesn't. 

"For those that have any doubts about this contract, if any rogues do not make the investments they are required to make, have no doubt that Argentina will be there to help," he said.

According to the terms of the contract, Argentina will pay Bolivia $5 per million British thermal units of gas through the first quarter of 2007, after which the price will fluctuate according to the market. 

At the signing ceremony in the eastern city of Santa Cruz, Morales thanked Kirchner for his nation's "permanent cooperation."

"When I had personal meetings with President Kirchner in Argentina, we would always encourage each other. He would say, 'Evo, we have to help Bolivia. These asymmetries, this inequalities from people to people, from state to state, have to end,'" said Morales, whose father once worked as a migrant laborer in Argentina. 

"My brother President Kirchner, with this contract we are rectifying that asymmetry between Bolivia and Argentina," Morales continued. "We have begun to truly change Bolivia."

Kirchner, meanwhile, reaffirmed his country's support for Morales' populist reforms. 

"I say to all our Bolivian brothers: hold fast to your dreams, hold fast to your hope," Kirchner said. "Know that that road is not easy, to construct a new reality where there is equality, where there is justice."

A jubilant crowd packed the stands of Santa Cruz's Gilberto Pareja Coliseum, waving the two countries' striped flags and chanting "Evo gets it done!" as the two presidents signed the deal at a table set up on a basketball court. 

Morales also used the moment to call for South American unity, reaching out to Brazil, with which Bolivia has struggled to reach a natural gas deal.

Since Morales nationalized Bolivia's petroleum industry on May 1, Bolivia has sought to raise the price it charges Brazil for gas, but months of talks have yet to produce a deal. 

"With Brazil, as with Argentina, we have an obligation to live together for our people, resolving the problem of our people, like a marriage without divorce -- because we need each other," Morales said.