US ambassador Susan Rice left the United Nations on Tuesday slamming the Security Council's failure to act over the worsening Syria conflict as a "moral and strategic disgrace."
But while Rice criticized Russia and China for their veto of resolutions on the Syria war, she said it was not inevitable that relations with Russia would continue to sour.
Rice, who moves to become President Barack Obama's national security advisor, said: "The repeated failure of the Security Council to unify on the crucial issue of Syria I think is a stain on this body and something that I will forever regret."
"The council's inaction on Syria is a moral and strategic disgrace that history will judge harshly," Rice told reporters after making her farewells to other UN envoys after four and a half years as US ambassador.
Russia and China have three times used their vetoes as permanent members of the Security Council to block western-proposed resolutions that would have increased pressure on President Bashar al-Assad.
Rice stressed that there were no sanctions or threats of force in the proposed resolutions which she called "very mild."
"Yet we have been paralyzed, and I don't know how in any circumstance one could ascribe that to a failure of US policy or US leadership, when the vast majority of the council was ready and willing to move ahead."
Russia and China accused the United States, France and Britain of only seeking regime change in Syria, where the United Nations says more than 93,000 people have been killed in the past 27 months.
Rice said tougher sanctions against Iran and North Korea had been a Security Council success and showed it was possible to work with Russia, which could become a key dossier in her new White House job.
At Rice's final closed consultations, Russia's UN ambassador Vitaly Churkin read out a mock council statement which "expressed relief" at Rice's departure, according to diplomats at the meeting.
Russia is a "complex" relationship, Rice acknowledged to reporters after the good natured farewell from her rival envoy.
"While there are certainly important points of divergence, and there have been points of friction and there undoubtedly will be in the future, I am not prepared to predict that that is inevitable," she said.
"On issues as important as Iran and North Korea, and many others, we have been able to find common ground and effect outcomes that have been beneficial," Rice said.
Samantha Power, a White House advisor, has been named as the new UN ambassador and is awaiting Congress confirmation.