In a series of tweets on his official Twitter account, Trump said: "On Monday, they dropped a drone flying over international waters."
"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," Trump tweeted. "10 minutes before the strike I stopped it."
Causing so many casualties, Trump tweeted, would not have been "proportionate to shooting down an unmanned drone."
He pointed out that the US military had intended to hit three different sites in Iran, without specifying their location or nature.
In a justification for his decision, the US president said the response was disproportionate to downing the drone, reiterating his firm stance on not allowing Iran to have a nuclear weapon by saying: "Iran can never acquire nuclear weapons."
He revealed that he was imposing tougher sanctions on Iran, without giving details, but stressed that Iran is now much weaker.
He stressed that he was not in a hurry to respond militarily to Iran, saying at the same time, "our army is ready."
The New York Times said earlier on Friday that Trump had agreed to a plan to strike Iranian targets, and then abruptly dropped implementation.
Source: Sky News