WASHINGTON — The U.S. bluntly accused Russia on Friday of hacking American political sites and email accounts in an effort to interfere with the upcoming presidential election.

In a one-two punch, the United States also directly accused Russia of war crimes in Syria.

Moscow dismissed the accusations, which significantly worsen U.S.-Russia relations that already had deteriorated to the lowest point in years. Russian officials called the hacking allegations “some kind of nonsense” and said the war crimes talk was simply an effort to divert attention from America’s own failure to uphold a cease-fire in the bloody war.

The White House declined to say whether the hacking accusation would trigger sanctions against Russia. A senior Obama administration official said the U.S. would respond “at a time and place of our choosing.”

The official said the public wouldn’t necessarily know what actions the U.S. has already taken or will take against Russia involving cyberspace. The official wasn’t authorized to comment by name and requested anonymity.

“This is a huge deal,” said Michael Morell, the former deputy and acting director of the Central Intelligence Agency. “I can’t think of any time in our history where we have blamed another government for trying to interfere with our elections. … This is unprecedented across the board.”

Late Friday, the U.S. Department of Homeland Security and the Office of the Director of National Intelligence accused Russia of hacking American political sites.

“These thefts and disclosures are intended to interfere with the US election process. Such activity is not new to Moscow,” a statement by the two agencies said. “The Russians have used similar tactics and techniques across Europe and Eurasia, for example, to influence public opinion there. We believe, based on the scope and sensitivity of these efforts, that only Russia’s senior-most officials could have authorized these activities.”

Kremlin spokesman Dmitry Peskov dismissed the U.S. statement but didn’t deny Russia’s involvement.

“Every day there are tens of thousands of attacks on Putin’s website. Many of the attacks can be traced to the U.S.,” Peskov was quoted as telling the Interfax news agency. “We’re not blaming the White House or Langley every time,” he added, referring to the Virginia city where the CIA is based.

Earlier Friday, Secretary of State John Kerry said that Russian and Syrian military strikes against civilians and medical facilities in Syria should be investigated as war crimes. The situation in Syria has dramatically worsened since the collapse of a U.S.-Russia-brokered cease-fire two weeks ago.

“These are acts that beg for an appropriate investigation of war crimes,” Kerry said. “They’re beyond the accidental now, way beyond, years beyond the accidental. This is a targeted strategy to terrorize civilians and to kill anybody and everybody who is in the way of their military objectives.”

Kerry on Monday had cut off diplomatic discussions between Russia and the US over Syria hours after Russian President Vladimir Putin suspended a U.S.-Russia agreement on disposing weapons-grade plutonium. On Wednesday, Putin suspended another agreement on research cooperation in the nuclear and energy sectors.

As for the hacking, federal officials are investigating cyberattacks at the Democratic National Committee and the Democratic Congressional Campaign Committee.

And election data systems in at least two states — Illinois and Arizona — also have been breached. Intelligence officials say some states have experienced scanning or probing of their election systems, which in most cases originated from servers operated by a Russian company.

The U.S. statement stopped short, though, in attributing the activity to the Russian government. And administration officials emphasized that it would be difficult for hackers to alter the results of the Nov. 8 election because each U.S. state runs its own election system.

Democratic Party officials learned in late April that their systems were attacked after discovering malicious software on their computers.

CrowdStrike, a cybersecurity firm that investigated, said one of the groups identified in the Democratic National Committee attack, dubbed Cozy Bear, had previously infiltrated unclassified networks at the White House, the State Department and the Joint Chiefs of Staff. Another group detected, called Fancy Bear, had targeted private and public sector networks around the world since the mid-2000s. Cozy Bear and Fancy Bear are linked to Russia.

Adam Meyers, CrowdStrike’s vice president of intelligence, said not only did the hackers’ programming code sometimes match code used in earlier hacks by Russia but the behavior matches the country’s decades-long efforts to sow instability in Europe and Eastern Europe — both on the ground and in cyberspace.

Meyers said Russian state-sponsored hackers — believed to be the military intelligence agency GRU — were not only targeting the DNC but also NATO and other governments, such as Romania, at the same time, pointing to a highly complex operation with many resources.

Caches of emails were also made public in recent weeks that detailed thousands of messages from Democratic operatives, as well as former Secretary of State Colin Powell. Those emails were published by a different source, called DC Leaks, which the cybersecurity firm ThreatConnect had linked to Fancy Bear.

Friday’s official accusation of Russian involvement adds to campaign-year fodder stoked this summer by Donald Trump, who encouraged Russia to find — and make public — missing emails deleted by Democratic opponent Hillary Clinton.

“Russia, if you’re listening, I hope you’re able to find the 30,000 emails that are missing,” Trump said in July. He referred to emails on Clinton’s private server that she said she deleted — because they were private — before turning other messages over to the State Department.

Associated Press writers Josh Lederman, Tami Abdollah, Jack Gillum and Eric Tucker in Washington and Jim Heintz in Moscow contributed to this report.