US President Donald Trump confirmed that he was aware of Israel's actions before they occurred, expressing a preference for negotiation over Israeli strikes against Iran, he told the US media on Friday.

Speaking to US TV network Fox News, Trump noted that "There were several people in the leadership of Iran who will not be coming back." Iranian media reported that Islamic Revolutionary Guard Corps Commander-in-Chief Major General Hossein Salami, Iran's armed forces chief of staff General Mohammad Bagheri and two nuclear scientists were among those allegedly assassinated in Israeli attacks, though official confirmation has yet to be provided.

Trump went to say that "hopefully they get back to the negotiation table" while insisting that "Iran cannot have a nuclear bomb".

The Israeli strike may change the dynamics of US-Iran nuclear negotiations that was scheduled for Sunday in Oman. Iran has repeatedly stressed that their nuclear program is not for weaponry, but for the right of Iranians to peaceful nuclear use.

However, US officials said the US will help defend Israel if needed.

Earlier, US Secretary of State Marco Rubio said the United States was not involved in Israeli strikes inside Iran in an effort to distance Washington from getting in between the two adversaries.

"Tonight, Israel took unilateral action against Iran. We are not involved in strikes against Iran and our top priority is protecting American forces in the region," Rubio said in a statement.

Trump had spoken to Israeli Prime Minister Benjamin Netanyahu several times recently, according to Fox News.

Earlier, Trump warned that there is a "chance of massive conflict" in the Middle East, confirming that an Israeli attack on Iran is "possible".

Speaking to reporters in the US on Thursday, Trump said he would "love to avoid the conflict" and suggested that the US would like Israel to hold off on plans to strike Iran's nuclear sites while Washington and Teheran continue their negotiations.

"I want to have an agreement with Iran. We're fairly close to an agreement … I'd much prefer an agreement," the US president said. "As long as I think there is an agreement, I don't want them (the Israelis) going in because I think that would blow it – might help it actually, but it also could blow it."

Trump skipped Israel in his first Middle East trip this year, unlike his tour in his first term.