President Evo Morales' latest anti-U.S. diatribe came in a speech to thousands of peasants in his political stronghold: The United States is sending soldiers disguised as students and tourists to Bolivia.

The accusation, rejected Wednesday as unfounded by the U.S. Embassy, comes as Morales faces attacks by political opponents for his cozy relationship with President Hugo Chavez's Venezuela, including accepting aid from that country's military.

It's not clear how many Venezuelan troops are in Bolivia, but Venezuelan pilots have been ferrying Morales around the country for the past two weeks in two loaned military helicopters as he campaigns ahead of July 2 elections for an assembly that is to retool Bolivia's constitution.

Morales' accusation also comes as Bolivia seeks to extend a preferential trade agreement that has been a big boost to South America's poorest country, helping Bolivia export $380 million in goods to the United States last year.

Vice President Alvaro Garcia Linera is expected to head to Washington next month to lobby for an extension of the Andean Trade Promotion and Drug Eradication Act, which expires Dec. 31.

Relations between the two countries have been frosty since Morales took office in January, and U.S. officials have said it's unlikely Bolivia will get an extension. Washington wants Bolivia to join Peru and Colombia in signing bilateral free-trade agreements.

During Morales' speech Tuesday in Cochabamba state, home of his political base, he mentioned that U.S. Ambassador David Greenlee had sought a meeting with him.

"He asked for a meeting. I don't know what he's looking to discuss. I'm not at all afraid of talking or perhaps he's angry," said Morales.

"But I also have the right to complain because U.S. soldiers disguised as students and tourists are entering the country," said Morales, a leftist Aymara Indian whose plan for Bolivia includes the nationalization of its natural gas industry.

Morales offered no evidence to back up the claim. His spokesman, Alex Contreras, said Morales would be providing evidence, though he did not say when.

The U.S. Embassy issued a statement calling Morales' accusation "unfounded."

"We reiterate once more that we are supporting Bolivian democracy in a consistent way," the statement said.

On Sunday, during a meeting with coca growers, Morales had uttered a phrase in the native Quechua language that may have irritated the U.S. ambassador.

"I shouted, 'Qausachun coca (long live coca!), wanuchun yanquis (die Yankees!),' and perhaps that could have angered him," said Morales. "If he complains, I, too, have the right to complain."

Morales often intoned the incendiary Quechua phrase in speeches during his years as head of the coca growers' union, a post he continues to hold today.

Coca is the basis of cocaine. But it is also a widely used stimulant with traditional medicinal and spiritual uses in Bolivia. Morales' government and Washington have been at loggerheads over his promotion of coca leaf for export in products including tea, toothpaste and shampoo.

Tuesday's remarks were Morales' second direct reference to the United States in recent days. Last week, he told a crowd that he was prepared to defend his revolution with arms against any U.S. threat.

Earlier this month, Morales said without offering specifics that the United States had tried to assassinate him in the past.

Morales' main political opponent, former president Jorge Quiroga, accused him this week of compromising Bolivia's sovereignty by inviting so many Venezuelan soldiers.

Venezuelan Embassy officials did not immediately return phone calls seeking comment on how many of the country's soldiers were in Bolivia.

Military cooperation with the United States, meanwhile, has ebbed. The U.S. Embassy would not specify how many Department of Defense employees it has in Bolivia, saying only that they number about a few dozen.

Morales has taken advantage of widespread resentment of the foreign policy of U.S. President George W. Bush's government to boost his populist agenda to fight poverty and hunger in Bolivia.

Morales nationalized the country's natural gas industry on May 1 and has vowed to turn over to landless peasants parcels owned by absentee landlords who have let them lie fallow.