The strains are becoming increasingly pronounced as Venezuela and Bolivia buy more Russian arms and Mr. Chavez pushes other regional leaders to condemn Colombia for renewed agreements with the U.S. military.

"We are asking the empire to remove its claws from Latin America," Mr. Chavez said, referring to the United States, earlier this month on his Sunday talk show, "Hello President." He accused Colombian President Alvaro Uribe of "serving the imperial policy of divide and rule" by inviting increased U.S. military access to Colombia, which Mr. Chavez asserted would "pose a threat to Venezuela and all of South America."

Until recently, the Venezuelan populist appeared to be toning down anti-U.S. rhetoric, heeding advice from center-left counterparts such as Brazilian President Luiz Inacio Lula da Silva who have sought to improve relations with the Obama administration.

But Mr. Chavez has become increasingly virulent following a coup in Honduras last month that toppled a protege, President Manuel Zelaya. Even though the Obama administration also condemned the coup, Mr. Chavez has accused Washington of backing the ouster and has expressed fears that U.S. allegations about Venezuelan involvement in drug trafficking and terrorism are preparations for a move against him.

A U.S. State Department fact sheet states that there are no plans to increase U.S. military personnel in Colombia, where U.S. deployments have been in "gradual decline" from a peak of 600 to 800 in 2004.

About 300 to 400 remain, according to the State Department. Between $40 million and $60 million has been allocated for "construction improvements" to Air Force bases covered by the new agreements, U.S. officials say. Colombia has received more than $5 billion in U.S. military aid since 2000, according to Reuters news agency.

According to Pentagon officials, U.S. service members and defense contractors will have access to seven more Colombian air and naval facilities in addition to bases in Tolemaida, Larrandia and Tres Esquinas, where Americans operate radar and intelligence-gathering aircraft and train Colombia's armed forces.

U.S. officials insist that the bases will remain under the control of the Colombian government, and Secretary of State Hillary Rodham Clinton has denied that the upgraded defense agreements are directed against any third country.

"We should all support each other by cooperating against narco trafficking and terrorism," she said at a joint press conference last week with Colombia's foreign minister.

Mrs. Clinton called the new security arrangements "an extension of existing agreements which go back three administrations" to when her husband, former President Bill Clinton, launched Plan Colombia.

Mr. Chavez, however, tried to use the new agreements to rally support against Colombia at a recent meeting of UNASUR, a regional group promoted by Brazil to coordinate security policies. Venezuelan allies Ecuador and Bolivia backed Mr. Chavez.

Ecuadorean President Rafael Correa, who assumed the rotating presidency of UNASUR, has shut down a U.S. naval facility in the port of Manta. He also has been locked in a dispute with Mr. Uribe since Colombian forces raided a Colombian rebel camp in Ecuador last year, killing a top rebel commander.

Meanwhile, Bolivian President Evo Morales declared that any Latin American president allowing U.S. bases is committing "treason."

However, the left-leaning rulers of Brazil, Argentina and Chile - which have tended to side with Mr. Chavez in past regional disputes with Washington - said they respect Colombia's "sovereign decision" so long as U.S. military personnel do not target other South American countries.

Brazilian Foreign Minister Celso Amorim said Mr. Lula has contacted Mr Obama to obtain "written guarantees" to this effect. Mr. Lula has scheduled another UNASUR meeting in Argentina for Friday in an effort to defuse the growing tensions.

"The main thing is not to let the issue [of the bases], although it's delicate, to cause setbacks in South American integration," said Marcelo Bauchman, Brazil's chief presidential spokesman. "President Lula wants South America to present a united front for a dialogue with Washington," he said.

Mr. Lula visited Bolivia on Saturday to try to persuade Mr. Morales to tone down his opposition to the bases. But he was met with cries of "death to the Yankees" at a peasant rally at which Mr. Morales said that Bolivians were "victims" of the U.S. military.

Mr Morales' close alignment with Mr. Chavez has created friction with other neighbors, including Paraguay, whose leftist president Fernando Lugo - a former Catholic priest once defrocked for socialist militancy - has expressed concern about a Bolivian arms deal with Russia.

Paraguayan Foreign Minister Hector Lacognata asked for a special meeting to "obtain information about the acquisition of defense elements by Bolivia" after Mr. Morales announced a $100 million credit from Moscow to buy arms.

According to Bolivian officials, planned purchases include fighter aircraft, helicopters and a $30 million Antonov presidential jet equipped with a bedroom suite, conference room and satellite communications.

Mr. Chavez has bought more than $4 billion in Russian armaments, including sophisticated Sukhoi Su-30 fighter bombers, submarines and 100,000 assault rifles. He announced further purchases of Russian tanks in response to Colombia's U.S. base agreement.

Mr. Morales has said he turned to Russia because the United States vetoed a recent sale of jet fighters to Bolivia by the Czech Republic, a member of NATO.

"We don't need the U.S. to fight drug traffic," he said atBolivia's July 24 independence day celebrations. "We can get the arms from Russia and China."