BUENOS AIRES (Reuters) - The head of the Organization of American States (OAS) has called for the release of jailed Argentine social activist Milagro Sala, according to a letter posted on his Twitter account on Tuesday.

A banner that reads "Freedom for Milagros Sala", " No to criminalization of the protest" hangs on fence near the Argentina's Government House (back) in Buenos Aires, January 28, 2016. REUTERS/Enrique Marcarian

The letter marked an escalation in international pressure on Argentina to release Sala, a leader of the Tupac Amaru social organization in Argentina’s northwestern Jujuy province.

Sala was detained in January on charges of “instigation to commit crimes and disturbances,” Amnesty International said, after protesting in front of provincial government offices.

According to local media reports, the Tupac Amaru organization is also accused of fraud related to a housing construction program it led, which Sala’s husband, Tupac Amaru member Raul Noro, has called a political persecution. Noro was arrested in July over allegations of embezzlement and fraud, and has since been released along with three other group members.

In the letter, addressed to Sala and dated Nov. 28, OAS secretary-general Luis Almagro said her detention was not justified because there was no risk Sala would flee or interfere with investigative processes, and said he was “convinced of the need for your immediate release.”

The Washington-based OAS, of which Argentina is a member, is the Western Hemisphere’s main multilateral organization and promotes democracy, human rights, security and development in its 35 member states.

International human rights groups like Amnesty International have come to Sala’s defense, calling on President Mauricio Macri to order her release.

Last month, the United Nations Working Group on Arbitrary Detentions said a string of “consecutive accusations” had been leveled against Sala to justify her continued incarceration, amounting to a denial of her due process rights, Amnesty International said.

In response, Jujuy governor Gerardo Morales said the UN’s claim was “supported by false premises,” while Argentina human rights secretary Claudio Avruj emphasized that the UN resolution was non-binding and that authority over the case ultimately lies with Jujuy province, according to local media reports.

In an interview earlier this month with Argentina’s La Nacion newspaper, Morales said Sala had “robbed the poor.”