US Homeland Security secretary Jeh Johnson has revealed hackers have probed the voting systems of many US states, but claims there is no sign they manipulated any voting data.
National security officials are investigating a number of possible leads suggesting that cyber criminals are trying to influence the November 8 presidential election, including by hacking into systems run by the Democratic National Committee.
Johnson said "malefactors" using cyber attacks to "attempt to disrupt the administration of our elections” was not a challenge "just in the future – they are here today".
"In recent months, malicious cyber actors have been scanning a large number of state systems, which could be a preamble to attempted intrusions," Johnson said in a statement.
"In a few cases, we have determined that malicious actors gained access to state voting-related systems. However, we are not aware at this time of any manipulation of data."
Johnson urged state and local election officials to seek cybersecurity assistance, noting that 21 states have already done so.
"We hope to see more," Johnson said.
Republican presidential candidate Donald Trump has repeatedly claimed that the US election system is "rigged", while top Democrats in Congress have charged that Russia is behind repeated attempts to access both party data and state voting systems.
The head of the Democratic National Committee, Donna Brazile, last month said the organisation had been hacked by Russian state-sponsored agents who were trying to influence the election.
Debbie Wasserman Schultz resigned as DNC chair on the eve of July's Democratic National Convention after WikiLeaks published a trove of hacked DNC emails that showed party officials favouring eventual nominee Hillary Clinton over senator Bernie Sanders during the party's nominating contests.