Washington: The US Defence Secretary says he did not see specific evidence from intelligence officials that Iran was planning to attack four US embassies.

US President Donald Trump referred to the plot in justifying the killing of Iran's top general Qassem Soleimani in an airstrike in Baghdad.

While Defence Secretary Mark Esper said he agreed with Trump that additional attacks against US embassies were likely, he told CBS that Trump's remarks about imminent attacks on four embassies, made to Fox News, were not based on specific evidence.

"What the president said was that there probably could be additional attacks against embassies. I shared that view," Esper said.

"The president didn't cite a specific piece of evidence."

When pressed on whether intelligence officers offered concrete evidence on that point, he said: "I didn't see one with regards to four embassies".

White House officials have claimed Soleimani was targeted because of an imminent risk of attacks on American diplomats and service members in Iraq and throughout the region.

US Defence Secretary Mark Esper has admitted there was no specific evidence of imminent embassy attacks. AP

Iran retaliated for the killing by firing 15 missiles at two airbases inside Iraq used by US troops. On the same day as that attack, Iran admitted to shooting down a Ukrainian Airlines passenger plane near Tehran in what it said was a case of "human error".

Trump said on Friday Iran probably had targeted the US embassy in Baghdad and was aiming to attack four US embassies before Soleimani was killed in an American drone strike on January 3.

"We will tell you probably it was going to be the embassy in Baghdad," Trump said in an interview on Fox News. "I can reveal that I believe it would have been four embassies."

Esper said in a separate interview on CNN that the administration had "exquisite intelligence" that a broader attack against multiple embassies was likely.

However that information could only be shared with the "Gang of Eight", a group of top congressional leaders who get briefed on sensitive information that the rest of Congress does not have access to.

Reuters