President Trump revealed a remarkable level of detail Friday about the tense moments leading up to his split-second decision to call off a retaliatory strike on Iran for the downing of a U.S. drone -- saying he was worried about the casualties and "didn’t think it was proportionate."

“I didn’t like it,” he said in an interview with NBC.

TRUMP CONFIRMS HE CALLED OFF RETALIATORY IRAN ATTACK '10 MINUTES BEFORE THE STRIKE'

The president said he quizzed his generals before the planes would have taken off on how many people would be killed, and was told approximately 150 Iranians could die.

“I thought about it for a second and said: You know what? They shot down an unmanned drone...and here we are sitting with 150 dead people that would have taken place probably within half an hour after I said go ahead,” he said.

The interview came hours after he tweeted at length about Thursday's decision-making, saying the U.S. was "cocked & loaded to retaliate" with plans to hit three sites, but he reversed course after asking military leaders about how many would be killed.

"... I asked, how many will die. 150 people, sir, was the answer from a General. 10 minutes before the strike I stopped it, not ... proportionate to shooting down an unmanned drone. I am in no hurry, our Military is rebuilt, new, and ready to go, by far the best in the world," Trump said.

A source had confirmed to Fox News earlier Friday morning that the administration made a last-minute decision to call off the retaliatory strikes against Iran. But until Trump's tweets, few details about the aborted mission and the circumstances that led to the reversal were publicly available. And it remained unclear Friday whether strike plans have been definitively shelved.

Senior administration officials said Friday that the White House is not backing off military options, and is “maintaining the option of military action at all times.”

The dramatic showdown comes as Trump wrestles with the dual priorities of keeping his campaign promise to remove America from military theaters abroad to focus on rebuilding at home, and projecting strength in the face of aggression from Tehran.

On Thursday, he described Iran's shoot-down of the U.S. drone over the Strait of Hormuz as a “very big mistake” and an unintentional error by “someone who was loose and stupid.”

Trump’s strike nix generated mixed reaction on the Hill, with hawkish Republicans expressing concern that a lack of response could make the U.S. look weak.

TRUMP SAYS IRAN'S US DRONE SHOOT-DOWN MAY HAVE BEEN 'MISTAKE,' BUT 'COUNTRY WILL NOT STAND FOR IT'

“To shoot down a $200 million plane the size of an airliner that could have easily had 35 people on it, there needs to be a response. Am I disappointed today? Yes,” Rep. Adam Kinzinger, R-Ill., told Fox News.

“Ultimately it will be ‘is there a reaction?’ And if there is I think that’s fine. But if there’s not a reaction and we think we can negotiate then I think it will be a bad move,” he said.

Asked if he feared the president could look weak, he said: “I think there’s certainly a risk, yes.”

Democrats, meanwhile, expressed concern about the president’s handling of the Iranian threat, and said that Congress must be consulted on any hostilities.

"The president seems to suggest that he found out about, apparently, the death toll if they had gone through with the strike moments before they conducted the strike. Isn't that something you think he should have known about beforehand?" House Speaker Nancy Pelosi, D-Calif., told reporters, before urging Trump to "take a deep breath and de-escalate."

Amid reports and speculation that Trump was caught between his own instincts and the more hawkish members of his team, a senior administration official told Fox News earlier Friday that there was “complete unanimity amongst the President’s advisors and DOD leadership on an appropriate response to Iran’s activities.”

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“The president made the final decision.”

Trump also used the standoff to again blast his predecessor Barack Obama’s nuclear deal with Iran, while claiming that sanctions imposed by Trump’s administration have weakened the Islamic regime. He also said more sanctions have been imposed.

"Sanctions are biting & more added last night. Iran can NEVER have Nuclear Weapons, not against the USA, and not against the WORLD!" he tweeted.

Fox News’ Lucas Tomlinson, Gillian Turner, Jared Halpern and Kelly Chernenkoff contributed to this report.