The U.S. FBI said on Monday it was investigating after Donald Trump's presidential campaign said its internal communications were hacked and the campaign blamed the Iranian government.

The former president said on Saturday that Microsoft had informed his campaign that Iran had hacked one of its websites. Trump said Iran was "only able to get publicly available information."

The FBI is also investigating an alleged hack targeting advisers to the campaign of President Joe Biden and Vice President Kamala Harris, The Washington Post reported on Monday.

The FBI began the investigation in June, when Biden was still running for president, suspecting that Iran was behind the attempts to steal data from two U.S. presidential campaigns, the newspaper said, citing sources.

The Harris campaign did not immediately respond to a request for comment. Harris became the Democratic presidential nominee after Biden withdrew his bid last month.

The Iranian government has denied that it hacked the Trump campaign.

Trump's campaign has pointed to a report on Friday by Microsoft researchers that indicated that Iranian government-linked hackers tried breaking into the account of a "high-ranking official" on a U.S. presidential campaign in June.

The report added that the hackers took over an account belonging to a former political adviser and then used it to target the official. It did not provide further details on the targets' identities.