The FBI is urging US election officials to increase computer security after it uncovered evidence that hackers have targeted two state election databases in recent weeks, according to a confidential advisory.

The warning was in an August 18 flash alert from the FBI's Cyber Division. Reuters obtained a copy of the document.

Yahoo News first reported the story on Monday, citing unnamed law enforcement officials who said they believed foreign hackers caused the intrusions.

US intelligence officials have become increasingly worried that hackers sponsored by Russia or other countries may attempt to disrupt the November presidential election.

Officials and cyber security experts say recent breaches at the Democratic National Committee and elsewhere in the Democratic Party were likely carried out by people within the Russian government.

Kremlin officials have denied the allegations of Moscow's involvement.

Concerns about election computer security prompted Homeland Security Secretary Jeh Johnson to convene a conference call with state election officials earlier this month, when he offered the department's help in making their voting systems more secure.

The FBI warning did not identify the two states targeted by cyber intruders, but Yahoo News said sources familiar with the document said it referred to Arizona and Illinois, whose voter registration systems were penetrated.

Citing a state election board official, Yahoo News said the Illinois voter registration system was shut down for 10 days in late July after hackers downloaded personal data on up to 200,000 voters.

The Arizona attack was more limited and involved introducing malicious software into the voter registration system, Yahoo News quoted a state official as saying. No data was removed in that attack, the official said.