WASHINGTON (Reuters) - The U.S. government for the first time on Friday formally accused Russia of a campaign of cyber attacks against Democratic Party organizations ahead of the Nov. 8 presidential election.

The headquarters of the Democratic National Committee is seen in Washington, U.S. June 14, 2016. REUTERS/Gary Cameron

“We believe, based on the scope and sensitivity of these efforts, that only Russia’s senior-most officials could have authorized these activities,” a U.S. government statement said on Friday about hacking of political groups.

“These thefts and disclosures are intended to interfere with the U.S. election process.”

U.S. intelligence officials concluded weeks ago that the Russian government was conducting or orchestrating cyber attacks against the Democratic National Committee and the Democratic Congressional Campaign Committee, possibly to disrupt or discredit the election, in which Democrat Hillary Clinton faces Republican Donald Trump.

A Kremlin spokesman called the U.S. allegations “nonsense”, the Interfax news agency reported.

On Saturday, the Russian Foreign Ministry said the U.S. accusations lacked any proof and were an attempt by Washington to fan “unprecedented anti-Russian hysteria”.

“This whipping up of emotions regarding ‘Russian hackers’ is used in the U.S. election campaign, and the current U.S. administration, taking part in this fight, is not averse to using dirty tricks,” Deputy Foreign Minister Sergei Ryabkov said on Saturday in comments posted on the ministry’s website.

The Obama administration’s decision to blame Russia for the attacks is the latest downward turn in U.S. relations with Moscow, which are under strain over Russia’s actions in Syria and Ukraine and in cyberspace.

Also on Friday, U.S. Secretary of State John Kerry said Russian and Syrian actions in the Syrian civil war, including bombings of hospitals, “beg for” a war crimes investigation.

In addition, a U.S. intelligence official said on Friday that Russia was moving short-range nuclear-capable missiles into Kaliningrad, a tiny Russian enclave between Poland and Lithuania, confirming Estonian news reports.

Clinton’s campaign chairman, John Podesta, said the public blaming for the hacks left one remaining question of “why Donald Trump continues to make apologies for the Russians”. Trump had previously expressed doubt about Russia’s involvement. In July, he suggested Russia should attempt to retrieve and publish emails from Clinton’s private server.

Trump’s campaign did not immediately respond to a request for comment.

Hours after the U.S. government’s accusation was levied, WikiLeaks posted hundreds of emails on its website purportedly hacked from Podesta’s private account.

CHANGE IN TONE

Until Friday, the Obama administration had avoided publicly singling out Russia in connection with the mounting civilian deaths in Syria or the cyber attacks.

The statement by the Department of Homeland Security and the Office of the Director of National Intelligence did not blame the Russian government for hacking attempts against state election systems, but said “scanning and probing” of those systems originated in most cases from servers operated by a Russian company.

However, a Department of Homeland Security spokesman said U.S. officials had concluded that the hacking attacks or probes of state voter registration systems were “consistent with Russian motivations”.

Concern has grown about the reliability of the U.S. voting system as a result of the breach, and Trump has called the system “rigged,” but without providing specific evidence.

U.S. intelligence officials have said there is no evidence that voting recording systems have been manipulated.

Identifying Russia as the actor behind the cyber attacks on political organizations falls short of more punitive measures the United States has taken against other countries for cyber intrusions.

Lawmakers of both political parties welcomed the formal accusation. Republican Senator Cory Gardner, chairman of the U.S. Senate Foreign Relations Subcommittee on East Asia, the Pacific and International Cybersecurity, said he planned to introduce sanctions legislation.

Earlier this year, a U.S. grand jury indicted seven Iranians employed by two Iran-based computer firms on charges of hacking into the U.S. financial sector. In 2015, Obama announced sanctions against North Korea for hacking into Sony Pictures.

In 2014, the United States charged five Chinese military hackers for economic espionage aimed at U.S. nuclear, metals and solar industries.

A senior U.S. official said the administration is considering other retaliatory steps against Russia, but he declined to identify them. Those steps may remain covert, the official said.

The Democratic National Committee publicly disclosed intrusions into its systems in June and held Russia responsible. Leaks of committee emails from pro-transparency group WikiLeaks soon followed, demonstrating what appeared to be favoritism for Clinton over another Democrat, Bernie Sanders, by committee chairwoman Debbie Wasserman Schultz. Wasserman Schultz, a Florida congresswoman, stepped down.

In Friday’s statement, the government said disclosures of emails by WikiLeaks and hacking entities known as DCLeaks and Guccifer 2.0 “are consistent with the methods and motivations of Russian-directed efforts”.

WikiLeaks has not identified the source of its leaks and criticized those who have claimed it was Russia. Guccifer 2.0 has identified itself as a Romanian hacker, but U.S. intelligence officials have concluded that Guccifer 2.0 and DCLeaks are both a front for Russian spy units.