To accept a bribe is clearly wrong and if you do not report this, you are in effect consenting to someone accepting bribes.
In the UK and many other jurisdictions, the legal situation is that while one is not legally bound to report such action, it would be part of your professional duty to report this up the chain of command in order to reach the appropriate command level to deal with this on an organisation-to-organisation level.
See 9 of Spades
David Whetham, ‘The Challenges of Ethical Relativism in a Coalition Environment’, Journal of Military Ethics, 7 (2008), p304.