Her comments came amid an emergency meeting of the UN Security Council to discuss the ongoing war that has spilled into its sixth year.
Addressing an emergency meeting at the United Nations in New York, the US ambassador has charged Russia's actions in Syria are "wasting" what is left of the country.
Samantha Power says Russia has had the power to stop the suffering in Syria for a long time but has chosen not to use it.
She told the UN Security Council Russia and Syria are waging an all-out air and ground offensive against opposition-held eastern Aleppo.
"What you can be sure, sure, that Russia will not do, today or any day, is accept any responsibility for its actions. What you can be sure that Russia will not do is tell the truth."
Ms Power has accused Russia of preparing air strikes against Aleppo even while its foreign minister was insisting Russia was committed to finding a peaceful end to the conflict.
But Russian ambassador Vitaly Churkin says bringing peace to Syria is "almost an impossible task."
He has accused opposition armed groups of sabotaging the recent ceasefire.
"Hundreds of armed groups are being armed. The territory of the country is being bombed indiscriminately. And bringing peace is almost an impossible task now because of this."
The United Nations' special envoy on Syria, Staffan di Mistura, says conditions in Aleppo have reached what he describes as "new heights of horror."
"We have seen pictures of reported use of incendiary bombs of such intensity that they light up the pitch darkness as though it was daylight."
Britain's ambassador to the United Nations, Matthew Rycroft, says the Syrian and Russian governments are pairing up to carry out war crimes against civilians.
He also accuses the Syrian government of having what he describes as a "sick bloodlust against its people."
"To cap it all, water supplies, so vital to millions, are being targeted, depriving water to those most in need. In short, it is difficult to deny that Russia is partnering with the Syrian regime to carry out war crimes."
When Syria's UN ambassador, Bashar Jaafari, was called to speak, representatives from the United States, Britain and France all walked out in protest.
Mr Jaafari insisted that, by supporting groups opposed to the Syrian government, Western nations are encouraging terrorism.
"Is it not about time that those who are on this council who continue to lie to themselves and to public opinion, to insist on the protection and supporting what you sometimes call the 'moderate opposition' ... all these armed groups are only many faces of one coin, namely terrorism, which boomeranged. It's backfired and attacked your capitals with their practices, terrorist actions and their extremist ideology."
The verbal fight comes as at least 23 Syrians were killed in renewed government air strikes on the contested city of Aleppo.
A ceasefire negotiated by the United States and Russia earlier this month is now almost completely destroyed.
That follows nearly two weeks of intense fighting and an attack on a UN aid convoy.
