Kamala Harris’s presidential campaign has confirmed it was warned by the FBI it had been targeted by a foreign influence campaign, triggering renewed fears over foreign interference in US elections.

The disclosure – reported by NBC – came after Donald Trump’s campaign claimed it had been hacked in an email phishing attempt, suspected to have been carried out by Iran.

The FBI has confirmed in a statement it is investigating foreign hacking attempts against the campaigns.

Harris’s campaign insisted on Tuesday its cybersecurity measures had prevented its systems from being hacked.

“We have robust cybersecurity measures in place, and are not aware of any security breaches of our systems resulting from those efforts,” the campaign told NBC.

But the confirmation of the FBI’s warning evoked memories of the 2016 campaign, when Russia was widely believed to have hacked the Democratic party’s email system in an effort to destabilise Hillary Clinton’s presidential bid and help Trump. The Russian hacking effort involved files being given to WikiLeaks, which subsequently published them.

This time, the bureau said it was investigating efforts to hack the Biden-Harris campaign in the period before Joe Biden withdrew from the presidential race – effectively handing the Democratic nomination to Harris, the vice-president – as well as that of Trump, according to the Washington Post.

Three Biden-Harris campaign staff members received spear phishing emails designed to look legitimate in order to give an intruder access to wider email communications, the Post reported.

It is unclear whether the phishing attempt was successful, and the response by the now-renamed Harris campaign suggested that the vice-president’s staff were confident the campaign had not been hacked.

By contrast, the Washington Post and two other US news outlets – Politico and the New York Times – reported last weekend receiving apparently authentic files stolen from the Trump campaign. A spokesperson for the campaign said on Saturday it had been hacked in June. It is unclear whether the stolen campaign files were a result of the apparently successful hacking attempt.

The Trump campaign is understood not to have reported the hack to the FBI, due to its suspicion of the agency.

The phishing attempt is understood to have also targeted Roger Stone, a long-time Trump ally who is now formally linked to his current presidential campaign.

“I was informed by the authorities that a couple of my personal email accounts have been compromised,” Stone told the Washington Post. “I really don’t know more about it. And I’m cooperating. It’s all very strange.”

US intelligence officials said last month that Iran was attempting to sow discord and undermine Trump’s attempts to recapture the White House. Iran has denied the accusations.

The renewed suspicions come amid speculation that the country’s Islamic regime is preparing a strike against Israel in retaliation for the assassination of Ismail Haniyeh, the Hamas leader, in Tehran last week.