Colombia's Congress has passed a constitutional amendment to give military courts greater jurisdiction over crimes committed by armed forces members.

Human rights groups say the legislation will mean many crimes going unpunished based on the checkered record of Colombia's military.

The law, given final approval in the Senate by a 57-7 vote, is widely considered a bid by President Juan Manuel Santos to placate military chiefs whose support he needs in peace talks with Colombia's largest leftist rebel group. Santos is expected to sign it this week.

The law says military courts cannot try cases of rights violations including forced disappearance, sexual violence, torture, forced dislocation and extra-judicial executions.

But rights activists say the law's imprecise language means hundreds of extra-judicial executions could be transferred military courts.