Trump says he stopped Iran strike with just 10 minutes to spare because he was concerned about casualties Iran shot down a U.S. drone on Thursday it claimed violated its airspace.

President Donald Trump on Friday reacted on Twitter to reports that he ordered a military strike on Iran for shooting down an American drone, but then reversed his decision after the plan was underway, saying he called off the attack with just 10 minutes to spare because he was concerned about potential casualties.

In a series of tweets, he said, "We were cocked & loaded to retaliate last night on 3 different sights when 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. Sanctions are biting & more added last night. Iran can NEVER have Nuclear Weapons, not against the USA, and not against the WORLD!"

Sources told ABC News the reversal Thursday night was against the advice of Secretary of State Mike Pompeo and National Security Adviser John Bolton.

The plan and reversal was first reported by The New York Times.

A senior level administration source briefed on the plan says it would have escalated the situation quickly had it been carried out. Officials feared the attack could have caused hundreds of civilian casualties.

Trump elaborated on his decision in an excerpt of an interview with NBC News' Chuck Todd that aired Friday afternoon.

Trump said "nothing was green-lighted until the very end because things change" and that he never gave a final warning.

He said planes were not yet in the air but as the plan got close to needing final approval -- "to a point you would not turn back, you could not turn back" -- he said he was asked: “Sir, we are ready to go, we would like a decision.”

"And I said, “I want to know something before you go. How many people would be killed, in this case Iranians?” he told Todd.

When told approximately 150 could die, Trump said he thought, "I didn't like it. I didn't think it was proportionate."

House Speaker Nancy Pelosi on Friday, answering a reporter's question, said she had not been informed of the impending strike.

Later, in a statement, she said, "We are in an extremely dangerous and sensitive situation with Iran. We must calibrate a response that de-escalates and advances American interests, and we must be clear as to what those interests are.

"During our meeting with the President at the White House, Congressional Leaders stressed the necessity that we work with our allies and not strengthen the hand of Iran's hardliners. Democratic Leaders emphasized that hostilities must not be initiated without the approval of Congress," Pelosi said.

Senate Minority Leader Chuck Schumer was not informed, either, a source said.

Iran shot down a U.S. drone early Thursday, claiming it had flown into the country's airspace. The U.S. government claims it was operating in international airspace.

On Friday, Iran’s semi-official Fars news agency reported that the Islamic Revolutionary Guard Corps’ aerospace commander, Brig. Gen. Amir Ali Hajizadeh, had told reporters that while Iran was intercepting the drone it shot down Thursday, there was an American P-8 spy plane flying nearby with 35 crew members on board that also violated Iran’s airspace. He said they could have shot it down, too, but did not.

On Thursday, Trump called the decision to shoot down the drone a "mistake" made by somebody in Iran he labeled "loose and stupid" when speaking to reporters following a discussion with Canadian Prime Minister Justin Trudeau.

The comments came following a morning meeting with his top national security advisers over the downing of what the U.S. military said was an unarmed and unmanned U.S. RQ-4A Global Hawk drone flying over the Gulf of Oman near the Strait of Hormuz.

Shortly before, Trump had tweeted that "Iran made a very big mistake" after a top Iranian commander warned Iran was "ready for war."

Top politicians from the Senate and House, including Pelosi and Schumer, met with administration officials about the downed drone Thursday afternoon.

"The president may not intend to go to war here, but we're worried that he and the administration may bumble into a war," Schumer said.

Pelosi called it a "dangerous situation" and cautioned the president that the U.S. "cannot be reckless in what we do."

Gen. Hossein Salami, commander of Iran's Islamic Revolutionary Guard Corps, offered a strongly worded threat to the U.S. after the drone was downed.

"Shooting down the American spy drone had a clear, decisive, firm and accurate message," he said, translated from Farsi. "The message is that the guardians of the borders of Islamic Iran will decisively respond to the violation of any stranger to this land. The only solution for the enemies is to respect the territorial integrity and national interests of Iran."

The United States called for closed consultations at the U.N. Security Council on Monday to discuss "the situation in the Middle East," a UNSC diplomat confirmed to ABC News. A second diplomat said they will specifically discuss Iran.

ABC News' Luis Martinez, Elizabeth McLaughlin, Somaye Malekian, John Parkinson, Mariam Khan and Conor Finnegan contributed to this report.