Washington (CNN) The Obama administration said Friday it was "confident" that Russia was behind recent hackings of emails about upcoming US elections in an attempt to interfere with the process.

The announcement marks the first time the US administration has officially accused Russia of hacking into US political systems. Earlier in the week, the two countries broke off formal talks about a ceasefire in Syria.

"We believe, based on the scope and sensitivity of these efforts, that only Russia's senior-most officials could have authorized these activities," the Department of Homeland Security and Office of the Director of National Intelligence said in a joint statement.

"The recent disclosures of alleged hacked emails on sites like DCLeaks.com and WikiLeaks and by the Guccifer 2.0 online persona are consistent with the methods and motivations of Russian-directed efforts," the statement added. "These thefts and disclosures are intended to interfere with the US election process. Such activity is not new to Moscow -- the Russians have used similar tactics and techniques across Europe and Eurasia, for example, to influence public opinion there."

The announcement was referring to the breach of Democratic National Committee emails and the sites of other Democratic Party-linked organizations disclosed over the summer.

Officials told CNN that Friday's announcement follows long deliberations within the Obama administration as to whether and when to take this step. They have had confidence on the assessment for some time, with the only question being whether to go public. Previously law enforcement and intelligence officials had pointed to Russia anonymously, with lawmakers among the few to go on record accusing Moscow of being behind the intrusions.

Russia, however, rejected the US accusations.

"This is some kind of nonsense again," said Russian presidential spokesman Dmitry Peskov, according to the Russian Interfax News Agency.

He added, "Putin's website is attacked daily by several tens of hackers. Many of the attacks are traced to US territory. However, we don't blame the White House or Langley each time this happens."

The administration's decision to finger Russia came amidst the collapse of the US-Russian relationship in other areas, such as Syria, removing incentives for the White House to remain silent on the matter.

On Friday, Secretary of State John Kerry went so far as to call for Russian and Syrian military strikes against civilians and medical facilities in Aleppo to be investigated as "war crimes."

In a further sign of the deteriorating US-Russia relationship, the Wall Street Journal reported later in the day that Moscow deployed nuclear-capable missiles in Kaliningrad. The Russian enclave sits next to US allies Lithuania and Poland, further boosting tensions between Russia and Eastern European countries now in the Western fold.

The administration, however, disputes that there was any delay in naming Russia, saying the announcement was made public as soon as the proper evidence to make an official attribution of responsibility was gathered.

"We also worked as quickly as possible to release as much information as possible in order to provide state and local officials sufficient time to fortify their infrastructure," a senior administration official told CNN.

Another senior administration official said that the major focus of making the announcement at this time was an effort to reassure the public that government is on top of the situation, that officials know who carried out the hacking and that the US political system can withstand the attacks.

The joint announcement Friday from the two agencies said the US wasn't yet ready to attribute blame for a series of additional hacks and attempted hacks of voter registration websites in several states.

But US intelligence and law enforcement officials told CNN that there's strong evidence Russian intelligence services are also behind the cyberattacks against state voter registration websites.

There has been debate within the intelligence community about whether to "name and shame" the Russians for the cyberattacks. Some in the FBI and Justice Department felt the evidence was strong enough to point the finger, but others in US intelligence agencies and the Office of the Director of National Intelligence felt it could encourage retaliation or the exposure of US intelligence operations.

Others in the White House worried about the political overtones over naming Russia, fearing it would be seen as an effort to help Democrat Hillary Clinton given the warm exchanges between Republican candidate Donald Trump and Russian President Vladimir Putin.

The attack on the Democratic National Committee exposed emails appearing to favor Clinton over opponent Bernie Standards exchanged between senior staffers and the DNC chair, Debbie Wasserman Schultz, who was eventually required to step down following their disclosure.

Hackers thought to be working for Russian intelligence have also carried out a series of cyber breaches targeting reporters at The New York Times and other US news organizations.

Democrats on Congress' intelligence committees were ahead of the administration by a few weeks in deciding to publicly point to Moscow, releasing a statement on September 22 that said they had "concluded" that Russian intelligence agencies were responsible for the intrusions.

Trump has voiced skepticism about whether the Russians are to blame for the hacks, saying at a recent debate that it could also be "somebody sitting on their bed that weighs 400 pounds."

The Clinton camp on Friday sought to turn the latest conclusion into a broader attack on Trump's friendly posture toward the country.

"The world now knows, beyond the shadow of any doubt, that the hack of the Democratic National Committee was carried out by the Russian government in a clear attempt to interfere with the integrity of our elections," said John Podesta, Clinton's campaign chair, in a statement. "The only remaining question is why Donald Trump continues to make apologies for the Russians."

Podesta himself became the latest Democrat to be a victim of hacking, when Wikileaks published his personal emails later Friday. He ascribed the attack to "the Russians in their quest to throw the election to Donald Trump," though US authorities have yet to determine who was behind the breach.

Some Republicans on Capitol Hill who have called for a tough stance against the Kremlin applauded the Obama administration decision but pressed for more confrontation.

"We cannot allow such actions to go unanswered," said GOP Rep. Michael McCaul, chair of the House Homeland Security Committee. "Vladimir Putin's regime has crossed a line, and he should know that the United States will not allow our political process or our future to be dictated by foreign adversaries."

"Today was just the first step -- Russia must face serious consequences," said Sen. Ben Sasse, Republican of Nebraska. "The United States must upend Putin's calculus with a strong diplomatic, political, cyber, and economic response."

One Democrat offered a less hawkish response.

Adam Schiff, the top Democrat on the House intelligence committee, called for the US to work with European allies to "develop a concerted response that protects our institutions and deters further meddling."

His counterpart on the Senate intelligence committee, Dianne Feinstein, called the hacking "intolerable."

She said that the administration's decision to officially acknowledge the Russian role after she and Schiff came forward to point the finger at Russia last month "conveys the seriousness of the threat."

CORRECTION: This story has been updated to reflect that Interfax is not a state-owned Russian news agency.