U.S. intelligence and law enforcement officials are warning that hackers with ties to Russia's intelligence services could try to undermine the credibility of the presidential election by posting documents online purporting to show evidence of voter fraud.

The officials, who spoke on condition of anonymity, said however, that the U.S. election system is so large, diffuse and antiquated that hackers would not be able to change the outcome of the Nov. 8 election.

But hackers could post documents, some of which might be falsified, that are designed to create public perceptions of widespread voter fraud, the officials said.

'Rigged': In the past week Donald Trump has claimed that the election is 'rigged' against him' US officials say Russian hackers could try and capitalize on his claims by faking voter fraud

They said that they did not have specific evidence of such a plan, but state and local election authorities had been warned to be vigilant for hacking attempts.

On Oct. 7, the U.S. government formally accused Russia for the first time of a campaign of cyber attacks against Democratic Party organizations to interfere with the election process.

U.S. officials familiar with hacking directed against American voting systems said evidence indicates that suspected Russian government-backed hackers have so far tried to attack voter registration databases operated by more than 20 states. Tracing the attacks can be difficult but breaches of only two such databases have been confirmed, they said.

The officials said there is no evidence that any hackers have succeeded in accessing equipment or databases used to record votes. Many states use systems that would be difficult to hack or defraud, including paper ballots which initially are tallied by machines.

U.S. elections are run by state and local officials, not the federal government. On Nov. 8, votes will be cast in hundreds of thousands of polling stations in 9,000 different jurisdictions, according to the National Association of Secretaries of State.

The U.S. officials declined to comment on Republican candidate Donald Trump’s contention that the election is being "rigged." Trump said in the third and final presidential debate with Democratic candidate Hillary Clinton on Wednesday night that he would not say until the election results were known whether or not he would accept the outcome.

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Trump and his campaign officials have repeatedly said that the potential for voter fraud remains high but they have not provided any evidence.

On Thursday, Trump said he would accept the results of the election "if I win." He said he reserved the "right to contest or file a legal challenge in the case of a questionable result."

Clinton supporters said Trump is unwittingly aiding an effort by Russian President Vladimir Putin to undercut the credibility of the vote. Washington and Moscow are at odds over several issues, from Russian involvement in the Ukraine conflict, the war in Syria and cyber attacks.