WASHINGTON (Reuters) - U.S. lawmakers demanded that technology firms do more to fight "shocking" foreign efforts to influence U.S. politics on Wednesday, a day after Facebook Inc FB.O identified a new influence campaign tied to November's elections and despite President Donald Trump's denunciation of the issue as a "hoax."

FILE PHOTO: Facebook logo is seen at a start-up companies gathering at Paris' Station F in Paris, France on January 17, 2017. REUTERS/Philippe Wojazer/File Photo

The Senate Intelligence Committee has called executives of Facebook, Twitter Inc TWTR.O and Alphabet Inc's GOOGL.O Google to testify on Sept. 5 "to hear the plans they have in place, to press them to do more, and to work together to address this challenge," Senator Mark Warner, the panel's Democratic vice chairman, said at a hearing.

“All the evidence this committee has seen to date suggests that the platform companies - namely Facebook, Instagram, Twitter, Google and YouTube - still have a lot of work to do,” Warner said.

Technology executives have traveled to Washington several times to testify in Congress over the past year, including 10 hours of questioning of Facebook Chief Executive Mark Zuckerberg over two days in April.

The committee has been looking into reported Russian efforts to influence U.S. public opinion for more than a year, after U.S. intelligence agencies concluded that entities backed by the Kremlin had sought to boost Republican Donald Trump’s chances of winning the White House in 2016.

Moscow denies involvement.

Facebook said on Tuesday it had removed 32 pages and fake accounts from its platforms in a bid to combat foreign meddling ahead of November’s U.S. congressional elections.

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Facebook stopped short of identifying the source of the misinformation. But members of Congress said it was clear Russia was involved.

‘CORE OF OUR DEMOCRACY’

“While it is shocking to think that foreign actors used the social networking and communication mediums that are so central to our lives in an effort to interfere in the core of our democracy, what is even more troubling is that it’s still happening today,” Senator Richard Burr, the committee’s Republican chairman, told the hearing.

Burr and several other senators said they were pleased Facebook had taken action.

Warner said cyber criminals who have been caught “were just the incompetent ones,” and expressed concern that the U.S. government was not well positioned to detect or counter influence operations.

Allegations of Russian involvement in Trump’s 2016 victory have dogged his presidency. He has come under fire for discounting the threat of interference in November’s impending elections, when his fellow Republicans’ majorities in the House of Representatives and Senate are at stake.

As senators gathered for the hearing, Trump took to Twitter to urge Attorney General Jeff Sessions to end a federal investigation into Russian interference in the 2016 election, which he described as a “TOTAL HOAX.”

Both Republicans and Democrats at the hearing sought to make clear that foreign influence efforts extended beyond U.S. elections, and affected many countries.

“This is going on everywhere. It’s not limited to politics,” Burr said.

Facebook and other technology firms have been on the defensive for many months over political influence activity on their sites as well as concerns over user privacy.