New York:US President Donald Trump says that he called off an aerial bombing of Iran when he learned that it would have killed 150 people and it would have been a disproportionate response to the downing of an unmanned American drone.

He was giving his version on Friday of why at the last minute he stopped an attack on Iranian targets. The action was leaked to the media by sources in his administration.

He tweeted: "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."

However, Iran is not entirely off the hook for the downing of an unmanned RQ-4 Global Hawk surveillance drone on Thursday.

"I am in no hurry, our Military is rebuilt, new, and ready to go, by far the best in the world," Trump said.

While hawks in his administration and his Republican party want to go to war with Iran, Trump has so far resisted them. He has been against foreign military entanglements by the US and one of his campaign pledges was to end them and bring US forces back home.

The US and Iranian versions of the shooting down of the drone in the Strait of Hormuz area vary: Iranians say that it was in their territory, but Washington maintains it was over international waters.

Trump told reporters on Thursday: "Iran made a big mistake. This drone was in international waters, clearly. We have it all documented scientifically not just words. And they made a very bad mistake".

The Islamic Revolutionary Guards Corps (IRGC) said the drone was brought down by its Air Force near the Kouh-e Mobarak region, which sits in the central district of Jask, after the unmanned plane violated Iranian airspace.

While threatening retaliation, Trump had also left room for not acting harshly or delaying action. He hinted that the Iranian leadership may not be behind it, saying: "I imagine someone made a mistake." He added that he thought it was someone "loose and stupid who did it".

Earlier this month two petroleum tankers were attacked in the Gulf. The US blamed Iran for the attacks, but Tehran has denied any involvement. Trump later downplayed the incident, trying to tamp down the calls for retaliation.

India, meanwhile, has launched Operation Sanklp in the Gulf to protect shipping on which it depends heavily for energy needs, according to the Indian Navy.

It announced on Friday that it has deployed a guided-missile destroyer, INS Chennai, and a patrol craft, INS Sunayna, in the Gulf.

About 20 percent of global oil production moves through the Strait of Hormuz.

Trump's red line for heavy retaliation appears to be the death of Americans in an Iran-linked attack - which Tehran has not crossed so far.

Trump, laying down proportionality as a factor, is a marked departure from previous administrations. Both the Bushes, senior and junior, caused tens of thousands of deaths in Iraq and Afghanistan as collateral damage in their wars.

Even former President Barack Obama's administration hundreds of civilian died in drone and aircraft bombings.

The current tension between the US and Iran began after Trump renounced the multinational agreement with Iran to stop nuclear proliferation. That pact was signed during Obama's time by the US along with the other permanent members of the United Nations Security Council and Germany.

Trump imposed sanctions on Iran and on trade with it by others, affecting India's oil purchases.

On Monday, Iran said that it had increased production of low-grade uranium and would exceed the limits set by the multinational nuclear agreement.

The same day, the US announced it was sending 1,000 troops to the region, where it had moved 1,500 troops, a navy ship and bombers and jets last month.

Trump's National Security Adviser John Bolton has been advocating a strong action against Iran. Earlier this week, Secretary of State Mike Pompeo reportedly told members of US Congress that Iran was linked to al-Qaeda creating a justification for war.

Putting pressure on Trump to act against Iran, Republican Senator Lindsay Graham said: "Iran needs to get ready for severe pain inside their country. Their capability pales in comparison to ours."

Another Republican Senator, Tom Cotton, in an interview to Politico called for bombing Iran.

But Democrat Speaker Nancy Pelosi, in a rare agreement with Trump, said: "I don't think the President wants to go to war. There's no appetite to go to war in our country."

While acknowledging that "high tension wires are up in the region" and the US has to be strong protect its interests, she said: "We started to lose credibility on the subject when we walked away from the Iran nuclear agreement."

IRGC chief, Maj Gen Hossein Salami, said on Thursday: "The downing of the American drone was a clear message to America... Our borders are our red line and we will react strongly against any aggression". But he also said that Iran did not want a war.