VIDEO-US officially blames Russia for political hacking attempts

Alexei Nikolsky | AFP | Getty Images

President Barack Obama (R) meets his Russian counterpart Vladimir Putin (L) in Los Cabos, Mexico, on June 18, 2012.

Putin denied the allegations despite U.S. officials and cybersecurity experts pointing to evidence that Russia was behind the security breach that resulted in the release of thousands of emails.

While this activity is "not new to Moscow," the U.S. intelligence community and Department of Homeland Security said it would be "extremely difficult" for hackers to actually "alter ballot counts or election results," because of the "decentralized nature" of the U.S. election system and the "number of protections state and local election officials have in place."

The Department of Homeland Security, however, urged election officials to remain vigilant and ask for cybersecurity assistance if needed.

Despite earlier reports that Russia was behind the hack attempts, some have expressed skepticism. For one, Donald Trump seemed to question those conclusions in the first presidential debate.

"I don't know if we know it was Russia who broke into the DNC," the Republican presidential candidate said when the first presidential debate turned to the topic of cybersecurity. "She keeps saying 'Russia, Russia, Russia,' and maybe it was. It could be Russia but it could be China, could also be lots of other people. It could be someone sitting on their bed that weighs 400 pounds."

Tensions between the United States and Russia have been high since Moscow annexed Crimea in 2014. And in more recent months, representatives for President Barack Obama and Putin have struggled to coordinate efforts in Syria, resulting in increased tensions.

In September, Secretary of State John Kerry announced an agreement with Russia in an attempt to implement a nationwide cease-fire in the war-torn country. Kerry, at the time, called the deal a potential "turning point" in the Syrian conflict.

But the State Department said Monday that Russia did not hold up its end of an agreement. The Kremlin has similarly blamed the U.S. for the collapse of the deal and strongly cautioned the U.S. against carrying out attacks on Syrian government forces.

— NBC News' Bob Windrem, Kelly O'Donnell, Tracy Snyder and Alexey Eremenko contributed to this report.