(CNN) -- In Colombia's decades-old war between the state, guerrillas and paramilitaries, one nearly invisible group of casualties has stacked up.
Women and girls have been the victims of sexual violence at the hands of all the parties involved in the conflict, and there is impunity for the perpetrators, Amnesty International said in a report released Wednesday.
Security forces, guerrillas and paramilitary forces alike participate in "widespread and systemic" sexual violence, the report states.
Since the 1960s, Colombia has been at war with the FARC -- the Spanish acronym for the Revolutionary Armed Forces of Colombia -- a guerrilla group.
The leftist rebel army claims to work as a bulwark against Colombia's ruling class, but has been criticized for losing its original ideology and funding much of its operations with money from the drug trade.
Another source of violence has been the paramilitary groups originally formed by the government and businessmen as a protection from left-wing armed groups. They have since gained control of regions and are notorious for "social cleansing," displacement and extrajudicial killings.
"Women and girls in Colombia are often treated as trophies of war. They are raped and sexually abused by all the warring parties as a way to silence and punish them," Susan Lee, Americas director for Amnesty International, said in a statement.
The government of President Juan Manuel Santos has made a commitment to address the issue, but the organization reports it has not seen evidence of real improvements in the area of human rights and sexual violence against women.
A government spokeswoman told CNN Wednesday that they were not immediately familiar with the report, but that a statement would be released at a later time.
Given a lack of statistics, these crimes are almost "invisible," the report says.
For example, the number of examinations into suspected cases of sexual violence increased from 12,732 in 2000 to more than 20,000 in 2010. But of those, only 109 were classified as being related to the conflict. Field interviews show that fear of reporting the crime hides the extent of the problem.
The Amnesty report includes a number of stories of victims culled from interviews.
"Yolanda," an 11-year-old girl living in the Arauca Department, in 2005 was repeatedly pestered by a soldier who wanted to have sex with her. When she refused, the soldier took her to where the army was camped out and raped her, keeping her captive until the next morning, the report states.
When her family complained, at first authorities did not believe the mother, and later, the soldier threatened the family. The girl's family ended up fleeing their home out of fear. The case has still not been resolved and the soldier remains in the army, the report states.
The reasons why the parties involved in the Colombian armed conflict target women are varied.
According to the report, the different groups use sexual violence against women "to exploit them as sexual slaves; to sow terror within communities and to make it easier for military control to be imposed; to force whole families to flee their homes and allow land to be appropriated; and to wreak revenge on adversaries."
Of the more than 20,000 cases of sexual violence reported in 2010, 84% were women, and 85% were under the age of 18, Amnesty found.
A majority of physically mistreated women -- 73% -- did not report the violence, the group's researchers found.
But even in cases where violence was reported, the alleged victims are often frustrated by authorities' lack of action. Colombian officials have allowed impunity to reign for decades, the report says.
In another case, 18-year-old "Daniela" and a friend were collecting firewood in the Bolivar Department, when a man with a knife threatened them and ordered them into the jungle.
He identified himself as a member of a paramilitary group, the report states. According to Daniela, the man forced the two girls to kiss before raping both of them. Her friend managed to escape and alerted authorities, but police passed the buck to military because it happened in their jurisdiction, the report recounts.
The army also refused to help, and a local women's group organized a search party to find the man. This forced the police and army to act, and they captured the man. The man was eventually convicted and sentenced to six years in prison, but his affiliation with the paramilitaries was never taken into consideration, the report states.
"The Colombian authorities must implement an action plan to end sexual violence, including measures to combat discrimination against women and girls, and to put a stop to the impunity which ensures that those responsible for such crimes are not brought to justice," Lee said.
The report was the culmination of a two-year investigation.