By Aracely Lazcano

EL PASO, Texas, April 12 (Reuters) - A U.S. appeals court ordered on Thursday that anti-Castro Cuban exile Luis Posada Carriles stay in jail while federal prosecutors decide whether to fight a judge's ruling to let him out on bail.

The former CIA operative, accused by Cuba and Venezuela in the bombing of a Cuban airliner in 1976, had posted $350,000 in bail and was close to being released when the 5th U.S. Circuit Court of Appeals issued an emergency stay, the U.S. Marshal's Service said.

Posada Carriles, 79, is being held while awaiting a May 11 trial on seven immigration fraud charges.

He has been in U.S custody since May 2005 after he entered the country illegally seeking asylum. The longtime foe of Cuban leader Fidel Castro is wanted in Venezuela and Cuba on charges he masterminded the 1976 Cuban airliner bombing that killed 73 people.

The U.S. government has refused to hand him over and tried to find a country that would give him asylum. It got no takers and instead has fought to keep him behind bars.

Over the objections of federal prosecutors, U.S. District Judge Kathleen Cardone said last week he could go free, with limitations, if he posted the $350,000 bail.

Prosecutors argue he may flee if he gets out, but Cardone said he was too old and his health too bad to pose a flight risk.

The appeals court granted a stay to give prosecutors time to "explore the possibility of appealing the court's decision." Posada Carriles has until Tuesday to respond to the order.

Posada Carriles, who was involved in the failed 1962 Bay of Pigs invasion against Castro, is also accused of directing 1997 bombings in Havana that killed an Italian tourist and of plotting to assassinate the Cuban leader at a 2000 summit in Panama.

Cuba and Venezuela say Posada Carriles, who is a naturalized Venezuelan, is a terrorist and accuse the Bush administration of hypocrisy in its "global war on terrorism" for charging him only with minor immigration crimes.

In a column published on Tuesday in the Communist Party newspaper Granma in Havana, Castro, who handed power to his brother Raul after stomach surgery in July, accused the U.S. government of protecting the man he called "the monster" because of his ties to U.S. intelligence services.

Many Cuban-Americans, an important voting bloc, consider Posada Carriles a hero for his anti-Castro efforts and have urged the United States to let him go.

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