The recent hack and release of thousands of damaging emails linked to the Democratic Party is thought to be the work of the Kremlin, out to manipulate the American presidential election, according to US security experts.

"I think we are probably in full-scale, 'Yes, this is Russia trying to influence the election so they can have the person they prefer at the top of the American government,'" Jason Healey, a director at the Atlantic Council who has worked on White House cyber defences, told The Hill. And the person the Russian leadership wants isn't Hillary Clinton.

An investigation by the security firm CrowdStrike tied the theft of 20,000 emails first discovered in June 2016 to Kremlin-backed hackers, though Russia — and hacker Guccifer 2.0 — denied it.

But when the emails, many of which deemed embarrassing to the leadership of the Democratic Party, were leaked just before the party's convention, hackers' likely intention to influence the election was revealed, note observers.

The explosive leak revealed that members of the party leadership had plotted to undermine Bernie Sanders' campaign as he and his supporters had long insisted.

The damaging information has dealt a blow to the party at the opening of the convention in Philadelphia, forcing Debbie Wasserman Schultz to announce her resignation as Democratic National Committee chair, who opted under pressure not to open the convention.

Vladimir Putin
Tensions between the West and Russia have increased to a new high.Sean Gallup/Getty Images

"The timing, the way this [email release] is being done, makes it obvious that this is an attempt to damage the Democratic presidential campaign," said Scott Borg, director of the nonprofit research institute US Cyber Consequences Unit.

A US official involved in the investigation told Reuters that the classified information collected on the hack so far "... indicated beyond a reasonable doubt that it originated in Russia," and that it has "all the hallmarks of a classic intelligence operation intended to damage a perceived adversary."

Five other officials linked to the investigation told the Daily Beast that the email theft by "Russian government hackers" was a "deliberate attempt to influence the presidential election in favour of Donald Trump."

DNC chairwoman resigns amid email leak revealing Bernie Sanders plot

The FBI has opened an investigation into the DNC hack "to determine the nature and scope of the matter," said a statement.

"A compromise of this nature is something we take very seriously, and the FBI will continue to investigate and hold accountable those who pose a threat in cyberspace."

Trump signaled a willingness to "get along" with Russian President Vladimir Putin early on in a primary debate, and Putin has deemed him a "brilliant and talented person."

"He says he wants to move on to a new, more substantial relationship, a deeper relationship with Russia — how can we not welcome that?" asked Putin in an interview in late 2015. "Of course we welcome that."

In addition, if Trump becomes president he intends to scale back US military activity and commitment abroad. He recently said he would only come to the aid of Baltic states in the event of a Russian invasion only if the smaller nations "fulfill their obligations to us," and pay a bigger share of the defence costs.

On the other hand, Putin has a frosty relationship with the Obama administration, which included Clinton as the former secretary of state.

The Clinton campaign has already characterised the email leak as a pro-Trump effort by Russia. "I don't think it's coincidental that these emails were released on the eve of our convention here, and I think that's disturbing," Clinton campaign manager, Robby Mook, told CNN.

Donald Trump Jr called Mook's comments "disgusting," and Trump himself, who tweeted that it was the "new joke in town."

Wikileaks, which published the emails, has also denied that the leak was designed to influence the election.