Samantha Power says new penalties on Tehran, as championed by Republican and some Democratic lawmakers, would "almost certainly end" nuclear talks between Iran and world powers.
And she says the proposed legislation would weaken existing sanctions on Iran by undermining international cooperation.
Power was speaking today at an event in Louisville, Kentucky, hosted by the Senate Majority Leader Mitch McConnell, a Republican.
She sought to stress commonalities: fighting Ebola, confronting terrorists, ending Burma's dictatorship.
On these, she said undercutting Obama's engagement strategy could set back US foreign policy goals.
