GENEVA (Reuters) - The United Nations warned on Tuesday that a proposed constitutional reform in Colombia could lead to soldiers and police being let off the hook for alleged war crimes committed during the country's long-running conflict.

The office of U.N. High Commissioner for Human Rights Navi Pillay called on President Juan Manuel Santos and the head of Congress Roy Barreras to "reconsider" their support for the reform.

The proposed changes to the military justice system come amid peace talks with FARC rebels aimed at ending five decades of war, in which tens of thousands of people have died and millions have been displaced.

"If adopted, this reform would seriously undermine previous efforts undertaken by the Colombian government to ensure that human rights violations, allegedly committed by members of the Colombian military and police forces, are duly investigated and perpetrators held to account," U.N. rights spokeswoman Cecile Pouilly told a news briefing in Geneva.

Under the proposed bill, war crimes and arbitrary detention carried out by the army or police would be tried in military courts. Determination of the crimes would be handled by a military body, "with the consequent risk of impunity", she said.

Crimes against humanity and most gross violations would be excluded from the military criminal jurisdiction, she said.

But a preliminary investigation phase, essential for clarifying facts and identifying suspects, would be conducted by military or police criminal justice institutions, rather than an independent evaluation by competent judicial authorities.

Independent U.N. rights investigators, who report to the U.N. Human Rights Council, warned last month that military and police forces suspected of war crimes could escape prosecution under the proposed constitutional reforms.

"Noting that this bill comes at a time when the government and the FARC guerrillas are in the middle of peace negotiations, we wish to stress that transitional justice mechanisms are available for Colombia to address the serious human rights and humanitarian law violations that occurred and continue occurring during the internal armed conflict," Pouilly said.

"Only justice can bring true peace and reconciliation."

(Reporting by Stephanie Nebehay; editing by Jason Webb)