The warning was made during a congressional hearing on Thursday by Director of National Intelligence James Clapper, National Security Agency Director Mike Rogers and Undersecretary of Defense for Intelligence Marcel Lettre.
They described Russia as a “major threat” to a wide range of US interests because of its sophisticated capabilities and "highly-advanced offensive cyber program.”
Clapper said "our assessment now is even more resolute than it was" on October 7 when the US government first publicly accused Russia of staging cyber attacks on Democrats.
US Defense Undersecretary for Intelligence Marcell Lettre II, Director of National Intelligence James Clapper and NSA Director Admiral Michael Rogers testify before the Senate Armed Services Committee on Capitol Hill January 5, 2017
US intelligence agencies say Russia was behind hacks into Democratic Party organizations and operatives before the presidential election to influence the outcome.
US President-elect Donald Trump has expressed skepticism about the findings, heading for a conflict over the issue with Democrats and some fellow Republicans in Congress.
Many US lawmakers are concerned about Moscow’s policies in Europe and the Middle East and distrust Trump's praise of Russian President Vladimir Putin.
American intelligence officials have said the Russian cyber attacks were aimed at helping Trump defeat his Democratic rival Hillary Clinton. Moscow denies the hacking allegations.
Trump and top advisers believe Democrats are trying to delegitimize his election victory by accusing Russia of helping him.
Last week, outgoing President Barack Obama ordered a series of economic sanctions against Russia, as well as expulsion of 35 Russian diplomats over the hacking allegations.
Emails allegedly stolen by Russia from the Democratic National Committee (DNC) and Clinton campaign chairman John Podesta were leaked to the media by WikiLeaks prior to the election, embarrassing the Clinton campaign.
In a tweet on Wednesday, Trump expressed doubt on Russia’s role in the leaks, writing: "(WikiLeaks founder) Julian Assange said 'a 14 year old could have hacked Podesta' - why was DNC so careless? Also said the Russians did not give him the info!"
During the hearing, Senator John McCain, who chairs the Senate Armed Services Committee, asked Clapper whether he believed WikiLeaks' Assange had put American lives in direct danger. He said he agreed. Clapper also said he did not think Assange had any credibility.
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