Iran has conveyed to Washington that it will respond firmly to the United States if it becomes directly involved in Israel’s military campaign, the Iranian ambassador to the United Nations in Geneva said on Wednesday.

Ali Bahreini told reporters that he saw the US as “complicit in what Israel is doing”. Iran would set a red line, and respond if the United States crosses it, he said, without specifying what actions would provoke a response.

Israel launched an air war on Friday after saying it had concluded Iran was on the verge of developing a nuclear weapon. Iran denies seeking nuclear weapons. US President Donald Trump called on Tuesday for Iran's "unconditional surrender".

Bahreini called Trump’s remarks “completely unwarranted and very hostile. We cannot ignore them. We are vigilant about what Trump is saying. We will put it in our calculations and assessments.”

The US has so far taken only indirect actions, including helping to shoot down missiles fired toward Israel. It is deploying more fighter aircraft to the Middle East and extending the deployment of other warplanes, three US officials said.

“I am confident that (Iran's military) will react strongly, proportionally and appropriately. We are closely following the level of involvement in the US ... We will react whenever it is needed,” he said.

Thousands of people were fleeing Tehran and other major cities on Wednesday, Iranian media reported, as Iran and Israel launched new missile strikes at each other.