(AFP) - Venezuela's government battled growing international isolation on Tuesday, after the UN slammed it for using "excessive force" against protesters and Latin American nations condemned it for breaking "democratic rule."

President Nicolas Maduro invited allied nations including Cuba and Bolivia to Caracas, where their representatives echoed his assertion that Venezuela is the target of a plot by the "imperialist" United States.

"Venezuela is the big prize, the jewel of the crown -- that's what they say in the corridors in Washington," Maduro told his allies, saying the United States has designs on Venezuela because of its oil and mineral wealth.

Meanwhile, Caracas stepped up moves against the opposition, whose protest movement appears to be losing steam as repression against it increases. Four months of street violence have left nearly 130 dead.

Venezuela's supreme court sentenced a prominent opposition mayor, Ramon Muchacho, to 15 months in prison and stripped him of his right to hold political office. He was convicted on charges of not preventing protests in his district in the capital.

And a loyalist "Constituent Assembly" -- a body with supreme powers installed last week after elections marred by violence and allegations of fraud -- passed a law creating a "truth commission" to prosecute opposition leaders.

Over the weekend, the assembly fired the attorney general, Luisa Ortega, who had broken ranks with Maduro to become an outspoken critic of the direction he was taking.

The UN rights office on Tuesday slammed "widespread and systematic use of excessive force and arbitrary detentions against demonstrators in Venezuela."

It highlighted the use of force "to instill fear, crush dissent, and to prevent demonstrators from assembling, rallying and reaching public institutions to present petitions."

UN chief Antonio Guterres expressed concern "that recent developments could lead to further escalation of tensions."

- Economic crisis -

Venezuela is already struggling under a dire economic crisis in which much of the population of 30 million is suffering shortages of food and medicine.

The hardening of the Maduro's stance against the opposition made negotiations and a peaceful solution appear less likely.

Fears the crisis could tip into far worse violence were stirred on Sunday when a group of 20 armed men led by two renegade officers raided an army base in the third city of Valencia, stealing weapons from its armory.

Venezuela's military was still searching for the group after two were killed and eight captured in the raid.

The country's defense minister and head of the armed forces, General Vladimir Padrino, insisted the attackers were "terrorists" acting with US backing and in no way represented a split within the military.

On Tuesday Padrino addressed the Constituent Assembly in uniform, after the military earned unanimous backing from the body. He stressed the "anti-imperialist" and "revolutionary" character of the armed forces.

Outside, police dispersed a small knot of protesters while pro-government militants prevented lawmakers from the rival, opposition-controlled parliament that shares the same Legislative Palace from entering.

In Peru, a dozen American nations -- including Latin American heavyweights Brazil, Mexico and Argentina -- and Canada issued a joint statement condemning "the break in democratic rule in Venezuela."

They said they would not recognize the Constituent Assembly.

That adds to an indefinite suspension of Venezuela from the South American trade bloc Mercosur, decided on the weekend.

Maduro on Tuesday called for a dialogue with his regional rivals.

"The right wing across Latin America has broken the rules of the game and our way of living side by side," he told the meeting of his allies in Caracas. "I think what we need is regional dialogue... in which Venezuela would be respected."

- US sanctions -

The United States has taken the unusual step of imposing sanctions on a head of state by targeting Maduro with a freeze of any US assets he might have.

The administration of US President Donald Trump, who called Maduro a "dictator," could add further sanctions.

US Vice President Mike Pence was to make a hastily organized visit to Latin American countries next week.

That trip could be linked to coordinating action on Venezuela, though a White House statement quoted Pence as saying only that he wanted to build on "our shared economic and security goals."

© Agence France-Presse