BRASÍLIA, Brazil — The Bush administration has grown increasingly alarmed by a series of weapons purchases by Venezuelan President Hugo Chávez that senior U.S. defense officials described as a "one-man arm's race" that could destabilize South America for decades.

Venezuela has confirmed it is buying 100,000 AK-47 rifles from Russia next month. But Pentagon officials said the number of weapons could reach 300,000 — for a nation whose national guard and army total 62,000.

U.S. intelligence reports conclude that the government of Chávez is in the process of a multibillion-dollar effort to buy new warships and as many as 50 Russian attack helicopters, senior defense officials told reporters on condition of anonymity. Chávez is discussing the purchase of 30 MiG-29 fighter jets, a senior defense official said.

Defense Secretary Donald Rumsfeld, who was traveling in South America yesterday, urged Chávez to reconsider the purchases.

"I can't imagine why Venezuela needs 100,000 AK-47s, and I just personally hope that it doesn't happen," Rumsfeld said in an appearance with Brazilian Vice President José Alencar. "And I can't imagine that if it did happen that it would be good for the hemisphere."

Bernardo Alvarez, the Venezuelan ambassador to the United States, said Venezuela was only buying arms to replace basic equipment used to outfit the Venezuelan armed forces. He said the long-planned program was a "sovereign decision," adding that U.S. officials had not sought an explanation.

"They seem to want to create confusion or suspicions about what we are doing," he said.

Bush administration officials speculated that Venezuela had undisclosed military ambitions or it planned to "leak" weapons to leftist guerrillas.

The rifles are of concern because it is the weapon used by the Revolutionary Armed Forces of Colombia, usually known as the FARC. The FARC has been battling the Colombian government for decades. The United States and Colombia have said that FARC traffics in drugs.

Chávez also plans to purchase dozens of MI-17 and MI-35 attack helicopters from Russia, four corvettes gunboats from Spain and Super Tucano aircraft from Brazil.

It's the second time in a week that Rumsfeld has expressed concerns about weapons in Latin America. On Monday, the Bush administration announced it was halting $2.3 million in military aid to Nicaragua until it destroys 1,000 SAM-7 missiles.

The suspension of military aid to Nicaragua came as Sandinista leader Daniel Ortega, the nation's former socialist president, is rallying political opposition to destruction of the missiles. Ortega has denounced the U.S. demands as an intrusion on Nicaraguan sovereignty.

Rumsfeld's comments on Venezuela came while in Brazil as part of a Latin America swing that took him to Argentina on Tuesday. The stops highlight his interest in the U.N. peacekeeping force in Haiti. All three countries have contributed to the force.

Later, Rumsfeld visited the Brazilian radar Amazon Surveillance System, which became fully operational last year and has cut flights from drug traffickers by about one-third.

Rumsfeld wraps up his visit today in Guatemala.

The U.S. lifted a 15-year-old suspension on $3.2 million in aid to Guatemala because of human- rights progress in the Central American nation, U.S. defense officials told a Reuters reporter flying with Rumsfeld. Officials said Secretary of State Condoleezza Rice had certified progress in Guatemala, where thousands died in a brutal, three-decade civil war.