Russia, China, and Iran are trying to manipulate American public opinion before the 2020 elections but none has succeeded in corrupting the online polling system.
Beijing has mainly used conventional media to push for certain policies, including trade, while Moscow and Tehran have been more active on social media platforms, a senior US intelligence official said on Monday.
Washington named the three countries for trying to interfere in the 2016 presidential elections and the 2018 midterms.
The official did not provide specific examples, saying that doing so could compromise efforts to stop them.
US President Donald Trump is expected to meet Chinese President Xi Jinping and Russian President Vladimir Putin at this month’s G20 summit in Osaka.
The meeting of 19 leading countries and the EU coincides with increased tension with Iran after the US accused it of attacking tankers in the Gulf of Oman and shooting down an American drone.
Mr Trump increased pressure on Monday by imposing sanctions on Iran’s supreme leader, Ayatollah Ali Khamenei, and eight senior military commanders.
A researcher at cybersecurity firm FireEye said Iranian hackers had increased attacks against the US in recent weeks.
Iran’s state-backed hacking group, APT33, has sent waves of spearphishing emails to American banks and government entities this month to try to gain access to their computer systems, said Benjamin Read, a senior manager at FireEye.
If successful, hackers could disrupt banks’ operating systems by wiping computers and hampering critical functions, Mr Read said.
He estimated that APT33 had launched attacks on hundreds of targets within dozens of organisations within the past two weeks.
Mr Trump last week abruptly cancelled planned air strikes against Iran in retaliation for shooting down the drone on Thursday.
He approved a cyberstrike on Thursday night that disabled Iranian computer systems used for rocket and missile launches, The Washington Post reported.
In 2011 to the start of 2013, hackers linked to the Iranian government launched cyberattacks on about 50 US financial institutions. The move followed the imposition of US sanctions on Iran’s government and a cyberattack on Iran’s nuclear programme, believed to have been carried out by America and Israel.
Yet the recent hacks against the financial sector are more sophisticated, Mr Read said.
The earlier Iranian hacks on American banks disrupted services by overwhelming computers with electronic communications, but the recent wave of spearphishing attacks would give hackers access to networks.