Russia may have stepped up efforts to meddle in the 2016 U.S. election after President Barack Obama's administration did not retaliate strongly to reports of its activities, former aides to the Democratic president told U.S. senators on Wednesday.

Victoria Nuland, a top State Department official dealing with Russia under Obama, told the Senate intelligence committee that Moscow seemed to have slowed its influence campaign after Obama made "a stern and personal warning" to Russian President Vladimir Putin in September 2016.

But she said the campaign seemed to accelerate during October, using social media platforms to spread false narratives as the Nov. 8 Election Day approached.