The Washington-based group Human Rights Activists said it had identified 239 of those killed in Israeli strikes as civilians and 126 as security personnel.
The group crosschecks local reports in the Islamic Republic against a network of sources it has developed in Iran.
Iran has not been publishing regular death tolls during the conflict and has minimised casualties in the past. Its last update, issued on Monday, put the toll at 224 killed and 1277 wounded.
Earlier, President Donald Trump said US patience was wearing thin but it has no immediate intention to "take out" Iran's leader while indicating he could dispatch diplomatic envoys.
Israeli Defence Minister Israel Katz said Iran's Supreme Leader Ayatollah Ali Khamenei could face the same fate as Iraqi President Saddam Hussein, who was toppled in a US-led invasion and eventually hanged after a trial.
"I warn the Iranian dictator against continuing to commit war crimes and fire missiles at Israeli citizens," Katz told top Israeli military officials.
Trump predicted that Israel would not be easing its attacks on Iran, however he might send US Middle East Envoy Steve Witkoff or Vice President JD Vance to meet Iranian officials.
Trump had said his early departure from a G7 summit in Canada had "nothing to do with" working on a ceasefire between Israel and Iran, and that something "much bigger" than that was expected.
In a further post on Tuesday, the US president said Khamenei's whereabouts were known but "we are not going to take him out (kill!), at least not for now," while adding: "Our patience is wearing thin."
Vance said the decision on whether to take further action to end Iran's uranium enrichment program, which some countries suspect is aimed at developing a nuclear bomb, "ultimately belongs to the president".
UK Prime Minister Keir Starmer said there was no indication that the US was about to enter the conflict.
Trump had not yet decided whether the US military will intervene on Israel's side, German Chancellor Friedrich Merz said.
Trump met for 90 minutes with his National Security Council on Tuesday afternoon to discuss the conflict, a White House official said.
Khamenei's main military and security advisers have been killed by Israeli strikes, leaving major holes in his inner circle and raising the risk of strategic errors, according to five people familiar with his decision-making process.
The Israeli military said Iran's military leadership was "on the run" and that it had killed Iran's wartime chief of staff Ali Shadmani overnight, four days after he had replaced another top commander killed in the strikes.
With Iranian leaders suffering their most dangerous security breach since the 1979 Islamic Revolution that toppled a US-backed monarch and led to clerical rule, the country's cyber security command banned officials from using communications devices and mobile phones, Fars news agency reported.
Israel had launched a "massive cyber war" against Iran's digital infrastructure, Iranian media reported.
Israel launched its air war on Friday after saying it had concluded that Iran was on the verge of developing a nuclear weapon.
Iran denies seeking nuclear weapons and has pointed to its right to nuclear technology for peaceful purposes, including enrichment, as a party to the Non-Proliferation Treaty.
Israel, which is not a party to the NPT, is the only country in the Middle East widely believed to have nuclear weapons.
Israel does not deny or confirm that.
Israeli Prime Minister Benjamin Netanyahu has stressed that he will not back down until Iran's nuclear development is disabled, while Trump says the Israeli assault could end if Iran agrees to strict curbs on enrichment.
with DPA and AP
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