Three days before the horrific chemical attack that killed nearly 1,500 people in Syria, American intelligence agencies had strong “indications’’ the atrocity was about to happen, according to a shocking report yesterday.

The disclosure in Foreign Policy magazine has infuriated leaders of the Syrian opposition, who said they were never warned.

“If you knew, why did you [the US] take no action?’’ an outraged Dishad Othman, a Syrian activist living in America, fumed to the magazine.

He said none of his compatriots in Syria had been given any warning.

The Obama administration revealed the startling news Friday.

It released an intelligence report that said, “In the three days prior to the attack, we collected streams of human, signals and geospatial intelligence that reveal regime activities that we assess were associated with preparations for a chemical-weapons attack.’’

Another Syrian activist, Razan Zaitouneh, told Foreign Policy, “it’s unbelievable they did nothing to warn people or try to stop the regime before the crime.

“Even the moment [of the gas attack], we thought it was as usual, limited and not strong,’’ she said.

That quickly changed “when we started to hear about the number of injuries.’’

US intelligence documents note that “satellite imagery, intercepted communications and social media reports’’ provided convincing evidence that the Aug. 21 attack was launched from government-controlled areas.

And it was aimed at rebel-held or contested areas, the magazine said.

The United States says the attack killed 1,429 people, including 426 children.

In the hours just after the attack was launched, intelligence picked up a panicked phone call between a Syrian Defense Ministry official and a leader of a government chemical-weapons unit, Foreign Policy said.

The minister, the magazine reported, demanded to know what had happened.

Administration officials have not addressed the question of what, if anything, they did to prevent the attack or warn victims beforehand.

The latest intelligence reports do not indicate whether the attack was ordered by top-ranking officials in Damascus or launched by a rogue military officer, the magazine said.

But President Obama’s secretary of state, John Kerry, has made it crystal clear that the administration is holding Syrian President Bashar al-Assad responsible for the attack.