WASHINGTON (Reuters) - 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.

FILE PHOTO: U.S. Assistant Secretary of State Victoria Nuland waves as she arrives at the Presidential Palace in Nicosia, Cyprus April 20, 2016. REUTERS/Yiannis Kourtoglou

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.

“I think it’s probably the case that the Russians expected deterrent measures and didn’t see them, and so felt they could keep pushing,” Nuland said.

The panel held the hearing as part of its investigation that began after U.S. intelligence agencies concluded that Moscow sought to influence the presidential campaign to boost Republican Donald Trump’s chances of winning the White House.

Russia denies seeking to influence the election. Trump has repeatedly dismissed investigations of Russian meddling as a “witch hunt.”

The committee’s Republican chairman, Senator Richard Burr, said he plans more hearings with Obama’s national security advisor, Susan Rice, and the former heads of the FBI and Department of Justice.

Nuland and Michael Daniel, who was Obama’s cybersecurity coordinator, said U.S. agencies and technology companies did not share enough information to effectively combat foreign election hacking.

“We didn’t have sufficient integration of information to understand how their campaign was structured,” Nuland said.

They said the threat from Russia to November’s U.S. mid-term elections and the 2020 presidential campaign remains.

Daniel said the administration was focused in 2016 on activity aimed at state and local election systems, and did not become fully aware until later of the extent of the foreign influence operations.

Nuland said the Obama administration held back from a strong response for reasons including a reluctance to seem to be influencing the contest between the Republican Trump and Democrat Hillary Clinton, Obama’s former secretary of state.

“There was already by late July and early August accusations by candidate Trump that the election would be rigged. And I think there was a concern that if this wasn’t handled properly, any move publicly would be seen as President Obama playing into those accusations,” she said.