US has 'credible evidence' Russia, Syria sanitizing alleged chemical attack site: State Department

The U.S. made the accusation almost a week after last Friday's bombing.

In addition to destroying evidence, she said, Russia and Syria are pressuring witnesses to change their stories.

"We have also watched as some people have seemingly been pressured by the government to change their stories about what actually occurred that night," Nauert added. "We have reports from credible people on the ground who have indicated that they have been pressured by both Russia and Syria to change their stories."

She couldn't say whether Russia and Syria had been "successful" in eliminating any trace of sarin or chlorine at the sites, but said it may mean that the OPCW investigation – if its team does get access – will not find any traces of the reported attack.

"It might be, it might be," she said – but she wouldn't "weigh into that conversation" any further and get ahead of the investigation: "We will have to wait and see."

Russian and Syrian responses to the U.S. accusation weren't immediately available.