Susan Rice has called the UN Security Council's inaction on the Syrian war "a stain" on the body, in final remarks as US ambassador to the organisation, BBC reported.
She criticised Russia and China for vetoing three resolutions that would have increased pressure on Syrian President Bashar al-Assad.
But the US envoy said the post had been "the best job I've ever had".
Ms Rice is leaving the ambassadorship to become President Barack Obama's national security adviser.
Mr Obama has nominated Samantha Power, a Pulitzer Prize-winning author and former White House adviser, to replace her. Ms Power must now be confirmed by the US Senate.