US officials have formally accused Russia of cyber attacks against political organisations in order “to interfere with the US election”.
Recent hacked emails are “consistent with the methods and motivation of Russia-directed efforts”, the Department of Homeland Security said.
Data revealing discussions within the Democratic Party was hacked earlier this year.
Some states reported “probing” attempts made on “election-related” systems.
However, officials said those attempts could not be directly linked to the Russian government.
Russian officials told Interfax news agency the claims it was involved in the cyber attacks were “nonsense”.
But a joint statement from the Department of Homeland Security and the Director of National Intelligence on Election Security said high-ranking officials at the Kremlin were almost certainly involved in the successful attacks.
“We believe, based on the scope and sensitivity of these efforts, that only Russia’s senior-most officials could have authorized these activities,” they said.
However, altering any actual ballots or election results would be “extremely difficult”, they added, because of a decentralised system and multiple checks and balances.
A number of embarrassing emails have come to light during the 2016 election campaign.