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

The statement was the first official accusation by Washington officials against Moscow in the spate of cyber attacks, although many analysts had said the hacks appeared to be from Russia.

The statement said the US intelligence community "is confident that the Russian government directed the recent compromises of emails from US persons and institutions, including from US political organisations."

It said the disclosures of alleged hacked emails on WikiLeaks and other sites and by the online persona "Guccifer 2.0" were "consistent with the methods and motivations of Russian-directed efforts."

"These thefts and disclosures are intended to interfere with the US election process," the statement said.

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"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."

But the US statement said that "it would be extremely difficult for someone, including a nation-state actor, to alter actual ballot counts or election results by cyber attack or intrusion."

That is because of the decentralised nature of the US election system, which is operated by thousands of state and local government entities.

"States ensure that voting machines are not connected to the Internet, and there are numerous checks and balances as well as extensive oversight at multiple levels built into our election process," the statement said.

"Nevertheless, DHS continues to urge state and local election officials to be vigilant and seek cyber security assistance from DHS. A number of states have already done so."

The White House declined to say whether the formal attribution would trigger sanctions against Russia.

The White House declined to say whether the formal attribution would trigger sanctions against Russia

The US will respond to the Russian cyber attacks "at a time and place of our choosing," a senior administration official said, adding such actions may be covert.

"We will take action to protect our interests, including in cyber space, and we will do so at a time and place of our choosing," the official said, echoing language usually reserved for military campaigns.

"The public should not assume that they will necessarily know what actions have been taken or what actions we will take."

The hacking claim was another setback in already strained US-Russia relations.

Earlier on Friday, US Secretary of State John Kerry called for an investigation into the Russian and Syrian regime bombardment of the city of Aleppo over potential war crimes.

"Russia, and the regime, owe the world more than an explanation about why they keep hitting hospitals and medical facilities and children," Kerry said.

"These are acts that beg for an appropriate investigation -  war crimes."

In response, Russia's foreign ministry spokeswoman Maria Zkharova said Kerry's comments were an attempt to distract from the collapse of the US-Russian brokered ceasefire, adding that there were legal consequences to such accusations, TASS news agency reported.

Assad's forces launched their assault on eastern Aleppo, home to around a quarter-of-a-million civilians, two weeks ago after a US-Russian brokered ceasefire fell apart.

Agencies contributed to this report.