Washington (CNN) Retired Adm. William McRaven, a former Navy SEAL who led US Joint Special Operations Command from 2011 to 2014, said Thursday that he has serious doubts about President Donald Trump's claim that he called off retaliatory strikes on Iran at the last minute because that is when he learned there would be casualties.

"We were cocked & loaded to retaliate last night on 3 different sights (sic) when I asked, how many will die. 150 people, sir, was the answer from a General ... 10 minutes before the strike I stopped it," Trump tweeted at the time.

McRaven, who oversaw the 2011 raid that killed Osama bin Laden, told reporters that he was ultimately happy with the President's last moment decision not to carry out the strikes after the downing of a US drone in June and agreed it would not have been a proportional response.

However, the retired admiral made it clear that he found it "hard to believe" Trump stopped the strike at the last minute upon learning that there would be casualties. Drawing on his experience in similar situations, McRaven said the casualty count is typically part of the detailed briefing given to the president and national security adviser when they first request military options.

"When I heard, after the fact, that it was only at the last minute that the President realized there would be casualties, frankly, I find that hard to believe because the casualty count is almost always part of the military's briefing when it comes to a strike on a target," McRaven told reporters during a call about a new report from the nonprofit International Crisis Organization focused on Iran.

Read More