Democratic US presidential candidate and former Vice President Joe Biden. Photo: Reuters / Kevin Lamarque.

State-backed hackers from China have targeted staffers working on the US presidential campaign of Democrat Joe Biden, a senior Google security official said on Thursday. The same official said Iranian hackers had recently targeted email accounts belonging to Republican President Donald Trump’s campaign staff.

The announcement, made on Twitter by the head of Google’s Threat Analysis Group, Shane Huntley, is the latest indication of the digital spying routinely aimed at top politicians.

Huntley said there was “no sign of compromise” of either campaign.

Iranian attempts to break into Trump campaign officials’ emails have been documented before. Last year, Microsoft Corp announced that a group often nicknamed Charming Kitten had tried to break into email accounts belonging to an unnamed US presidential campaign, which sources identified as Trump’s.

Earlier this year, the threat intelligence company Area 1 Security said Russian hackers had targeted companies tied to a Ukrainian gas firm where Biden’s son once served on the board.

Google declined to offer details beyond Huntley’s tweets, but the unusually public attribution is a sign of how sensitive Americans have become to digital espionage efforts aimed at political campaigns.

“We sent the targeted users our standard government-backed attack warning and we referred this information to federal law enforcement,” a Google representative said.

Hacking to interfere in elections has become a concern for governments, especially since US intelligence agencies concluded that Russia ran a hacking and propaganda operation to disrupt the American democratic process in 2016 to help then-candidate Trump become president. Among the targets was digital infrastructure used by 2016 Democratic presidential candidate Hillary Clinton’s campaign.

Moscow has denied any meddling.

Attempts by foreign adversaries to break into presidential campaigns are commonplace but the unusually public attribution offered by Google is a sign of how sensitive Americans have become to digital espionage efforts aimed at candidates.

“We are aware of reports from Google that a foreign actor has made unsuccessful attempts to access the personal email accounts of campaign staff,” a Biden campaign spokesman said. “We have known from the beginning of our campaign that we would be subject to such attacks and we are prepared for them.”

The Trump campaign, the Chinese Embassy in Washington and the Iranian United Nations Mission in New York did not immediately respond to requests for comment.

Charming Kitten, the group identified by Google as being responsible for the targeting of the Trump campaign, has also recently hit the headlines over other exploits, including the targeting of the pharmaceutical company Gilead Sciences Inc.

Earlier this year, Reuters tied the group to attempts to impersonate high-profile media figures and journalists.

John Hultquist, senior director of intelligence analysis with US cybersecurity firm FireEye Inc , described the two hacking groups as “espionage actors” and said they were likely attempting to collect intelligence rather than steal material to leak online.

The FBI and Office of the Director of National Intelligence both declined to comment.