BOGOTA, Colombia (CNN) -- Colombia's U.S.-backed security forces are engaging in "systematic and widespread" extrajudicial executions of innocent civilians as part of their counterinsurgency campaign, a top United Nations diplomat said Saturday.

Speaking in Bogota after a weeklong fact-finding tour, Navi Pillay, the U.N. High Commissioner for Human Rights, said the scale of the killings could constitute a "crime against humanity" under international humanitarian law, adding that international courts could intervene if the Colombian government was "unwilling or unable" to handle the investigations itself.

"An offense becomes a crime against humanity if it is widespread and systematic against the civilian population," Pillay said at a news conference.

"We are observing and keeping a record of the number of extrajudicial killings [in Colombia], and it does appear to be systematic and widespread in my view," Pillay said. Watch Pillay explain the investigation process »

The goal, she added, is to have the national authorities investigate these crimes "and prosecute the perpetrators. It's only when a country is unable and unwilling that the International Criminal Court, for instance, would have the power to intervene."

Her comments come three days after Colombian President Alvaro Uribe fired 25 army officers and non-commissioned officers, including three generals and 11 colonels, for alleged involvement or negligence in a case involving the forced disappearance and summary executions of at least 11 young men from a poor Bogota suburb this year.

It was the biggest purge in Colombian military ranks for alleged human rights abuses.

Although Pillay welcomed the move, she said Saturday that she hoped it would be the start -- not the end -- of a thorough process to improve the human rights record of the Colombian military.

U.N. officials say they don't keep comprehensive statistics of forced disappearances and summary executions blamed on the security forces. But a U.N. source said the organization received between 200 and 300 such complaints every year.

A U.N. report published last year said the organization had seen "significant increases" in the number of cases.

Meanwhile, the government attorney general's office says it opened close to 800 investigations into accusations of summary executions by the police and military between January 2003 and September 2007.

Typically, according to the U.N. and the attorney general's office, security forces will "disappear" or kill civilians and later present them to the media as leftist rebels or right-wing paramilitary fighters killed in combat.

Since 2000, Colombia has received about $5 billion in mostly military aid from the United States to fight drugs and the guerrilla war.

Under the terms of the aid package, Washington is supposed to thoroughly vet the human rights record of Colombian military units. If abuses are uncovered, Washington can suspend aid to the offending units.

U.S. authorities have not publicly said whether aid will be affected by the latest investigations and subsequent purge of the military high command.

None of the commanders fired earlier this week has been arrested or charged with any crime. But Uribe has said criminal investigations are ongoing and promised that offenders would be jailed.

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