Susan Rice has departed her post as US ambassador to the organisation with regret that more was not done by the Security Council to stem the bloodshed in Syria.
She criticised Russia and China for vetoing three resolutions that would have increased pressure on Syrian President Bashar al-Assad.
Ms Rice is leaving the ambassadorship to become President Barack Obama's national security adviser.
The BBC reports Samantha Power, a Pulitzer Prize-winning author and former White House adviser, has been nominated to replace her. Ms Power must now be confirmed by the US Senate.
Ms Rice described her time at the UN as "a remarkable period", but said she regretted more was not done to stem the bloodshed in Syria.