Valentina Rosendo Cantu, a Me'phaa indigenous woman, waged a 16-year legal battle to make the soldiers face justice after they detained, tortured and sexually assaulted her during an anti-drug trafficking operation in the southern state of Guerrero.

"Everywhere I turned, public officials mocked me, from the moment I filed my complaint," she said at a press conference announcing the sentence.

The soldiers detained Rosendo Cantu for questioning about the whereabouts of another person during an operation to combat opium poppy farming in February 2002.

They then threatened, beat and raped her, according to her testimony, which led to a guilty verdict against the Mexican government at the Inter-American Court of Human Rights in 2010.

That ruling also ordered that the soldiers should face trial in a Mexican court, which yielded the sentence announced on Monday.

"This is a very important sentence (and)... an example that the courts can contribute to changing the reality of this country," said Jan Jarab, the Mexico representative of the United Nations High Commissioner for Human Rights.

The Mexican army and police have been accused of numerous human rights violations in recent years as they fight the country's powerful drug cartels.

Last month, the UN said there were "strong indications" the security forces were responsible for a wave of kidnappings and suspected killings in the city of Nuevo Laredo, on the border with the United States.

The Mexican marines announced Monday they had allowed state and federal prosecutors and the National Commission on Human Rights access to their bases to investigate the cases.

At least 21 people have disappeared in the city since February, often detained by uniformed officers in the middle of the night and never heard from again, according to the UN.

Family members searching on their own have found the bodies of at least six of the victims.

The navy said in a statement it had "made it clear to the prosecutor general's office its full willingness to cooperate to the fullest" in the investigations.