The US Federal Bureau of Investigation (FBI) urged state officials to boost election security following cyber breaches of two states' elections systems, Yahoo News website reported on Monday.

The FBI's cyber division issued a confidential "flash" alert to state officials on August 18 saying it launched an investigation in light of evidence that hackers breached voter data systems in two states.

An FBI spokeswoman said on Monday that the bureau does not comment on specific alerts, but noted that it "routinely advises" on "various cyber threat indicators observed during the course of our investigations."

The warning came days after Homeland Security Secretary Jeh Johnson hosted a call with state election officials to discuss cyber security and election infrastructure.

While the Department of Homeland Security (DHS) is not aware of a particular cyber threat to voting systems in the US, it is "critically important" to secure election infrastructure amid a rapidly changing security landscape, the DHS said in a summary of the call.

US officials have been on edge following a series of cyber attacks on federal targets, including the Internal Revenue Services (IRS) in 2015.

In July, whistle-blowing website WikiLeaks published thousands of emails that showed a concerted effort by senior officials of the Democratic National Party to undermine US Senator Bernie Sanders' campaign for the White House.

Democratic presidential nominee Hillary Clinton's campaign, along with cyber security experts, has claimed that Russian hackers likely working on behalf of the government breached the party's databases and accounts. However, the Kremlin has denied the allegations.

Senate Minority Leader Harry Reid on Monday asked the FBI to investigate the threat of Russian interference in the US elections, including the possibility of falsifying election results.

"Evidence of a direct connection between the Russian government and Donald Trump's presidential campaign continues to mount," Reid said in a letter addressed to FBI director James Comey.

Republican presidential candidate Donald Trump in July called on the Russian government to find emails stored on Clinton's private server during her time as the US' top diplomat after the Democratic Party leaks.

ls/gsw (AP, Reuters)