By Theodore Schleifer

CNN

WASHINGTON (CNN) -- Nikki Haley, the US envoy to the United Nations, is characterizing Russia as skittish and isolated in the wake of Syrian leader Bashar al-Assad's deadly chemical weapons attack last week that killed scores of civilians, including children.

On Tuesday, US officials charged that Russia was trying to cover up the attack, and Haley said in an interview with CNN's Jamie Gangel that she thinks that Moscow "knew" about it beforehand.

"I think that if you look at the fact that when this information came out, they were so quick to defend," Haley said. "They didn't look shocked, they didn't look surprised. They were so quick to defend. And then the evidence comes out and we see exactly what it is. And we know exactly what the environment was. Then you realize that ..."

"They knew what was going on?" Gangel asked.

"I think that they knew, yes," Haley said.

"I think they're nervous," Haley continued. "I think that they very much feel as if they've been weakened by Assad's actions and their cover up. They realize the international community didn't buy it and is not buying it. And they now have to figure out how to save face. We're giving them all the options to save face."

Haley's comments come before critical meetings between Secretary of State Rex Tillerson and his Russian counterpart, Sergey Lavrov, which are now expected to focus on Assad's use of chemical weapons on his own people. A senior administration official confirmed to CNN Tuesday that the US has biological samples that confirm the use of sarin in last week's attack.

In an interview on Fox Business Network to air Wednesday, President Donald Trump said Russian President Vladimir Putin was backing "a truly evil person."

"And I think it's very bad for Russia," Trump said. "I think it's very bad for mankind. It's very bad for this world. But when you drop gas or bombs or barrel bombs -- they have these massive barrels with dynamite and they drop them right in the middle of a group of people -- and in all fairness, you see the same kids --- no arms, no legs, no face --- this is an animal."

Haley has emerged as the leading hawk in the Trump administration's foreign policy firmament and has been far more critical of Russia and Syria than other national security figures in the White House. Haley and the White House now say Assad must step down in order to create a stable Syria, a position the administration did not hold prior to Assad's use of sarin gas in the attack last week.

Haley, however, would not offer a timeline for Assad's ouster.

"Russia is on an island, and they now have to decide whether they're going to stay on an island and lose anyone that's willing to work with them, or whether they're going to come back -- and not lose face -- and actually try and do something constructive," she said.

Haley reiterated that the administration would not commit to future military action there, but also did not rule it out.

The former South Carolina governor also weighed in on comments by White House press secretary Sean Spicer, who used Adolf HItler in a criticized analogy to describe the heinousness of Assad's actions.

"It's unfortunate," she said. "We should never have comparisons with Hitler, ever. What I hope Sean was trying to say is just how awful Assad is."

CNN's Joe Johns contributed to this report.