Trump said the plan was to hit three different sites in response to the drone's downing on Thursday, which Tehran said took place over its territory and which Washington said occurred in international airspace over the Strait of Hormuz.
The drone incident aggravated fears of direct military confrontation between the longtime foes, and oil prices rose a further $1 per barrel to above $65.50 on Friday due to worries about possible disruptions to crude exports from the Gulf.
In a series of early morning tweets, Trump said he was in no hurry to launch a strike and that U.S. economic sanctions designed to force Iran to curb its nuclear and missile programs and its involvement in regional wars were having an effect.
He also said the United States imposed additional sanctions against Iran on Thursday night following the destruction of the Global Hawk drone by an Iranian surface-to-air missile, but it was not immediately clear what those economic penalties may have been.
"Ten 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 tweeted.
....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!— Donald J. Trump (@realDonaldTrump) June 21, 2019
White House national security adviser John Bolton, Secretary of State Mike Pompeo and CIA Director Gina Haspel, along with the rest of Trump's team, favored a retaliatory strike, said a Trump administration official who asked not to be named.
Earlier on Friday, Iranian officials told Reuters that Tehran had received a message from Trump warning that a U.S. attack on Iran was imminent but saying that he was against war and wanted talks on a range of issues.
News of that message, delivered through Oman overnight, came shortly after the New York Times reported that Trump had called off air strikes targeting Iranian radar and missile batteries at the last minute.
"In his message, Trump said he was against any war with Iran and wanted to talk to Tehran about various issues," one of the officials told Reuters, speaking on condition of anonymity.
"He gave a short period of time to get our response but Iran's immediate response was that it is up to Supreme Leader (Ayatollah Ali) Khamenei to decide about this issue."
A second Iranian official said: "We made it clear that the leader is against any talks, but the message will be conveyed to him to make a decision.
"However, we told the Omani official that any attack against Iran will have regional and international consequences."
Khamenei has the last say on all state matters and has ruled out any talks with Washington while Tehran is under sanctions.
Iran shot down the drone after weeks of festering tension amidst a spate of attacks on oil tankers in the Gulf region.
In his initial response on Thursday, Trump said he was not eager to escalate a stand-off with Iran over its nuclear and ballistic missile activities and support for proxies in various Middle East conflicts.
He said the unmanned drone might have been shot down in error by someone who was acting "loose and stupid", though added: "This country will not stand for it."
In a sign that the United States may be interested in pursuing diplomacy for now, it asked the U.N. Security Council to meet on Iran behind closed-doors on Monday, diplomats said.
"We will brief the council on the latest developments with regard to Iran and present further information from our investigation into the recent tanker incidents," the U.S. mission to the United Nations said in a note to council colleagues, seen by Reuters.