Caracas, Venezuela - Venezuela announced an investigation Monday into claims of torture of migrants who the US sent to a notorious prison in El Salvador, from where they were freed last week.

"We have decided to open a formal investigation," Attorney General Tarek William Saab told reporters and called on "the International Criminal Court, the UN Human Rights Council, and relevant organizations both in America and the world, to do the same, to act accordingly."

A total of 252 Venezuelan men swept up in US President Donald Trump's immigration dragnet were returned home from El Salvador last Friday after their families had fretted for months without news about their welfare.

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Saab said dozens of officials in the prosecutor's office have been interviewing the migrants, who have told of sexual abuse, daily beatings, and rotten prison food.

"They made serious allegations of massive violations of their human rights," he said.

The men were accused in the US – without evidence – of being gang members and flown to El Salvador in March, and locked up in the CECOT prison built to house dangerous gangsters.

There, they were shackled, shorn, and paraded before cameras – drawing an international outcry.