Authorities fear that ISIS will launch an “on-camera slaughter of major proportions” after kidnapping at least 300 Christians in Syria last week, a top US official told The Post.

“What is very disconcerting is that they rounded up all of these people, and we know from experience that they usually kill their captives on film and in the most brutal fashion,” the official explained.

“Remember, they’ve already gone from beheading one person on camera to, most recently, beheading 21 people on camera,” the official added, referring to the nearly two dozen Coptic Christians slaughtered on video released last month.

“This could very well turn into an on camera slaughter of major proportions.”

The terrorists have garnered the ire of Pope Francis, who has called the 21 killed Christians “martyrs’’ — and led a massive pray-in at St. Peter’s Square in Rome on Sunday for others held captive.

“Everyone, in line with their possibilities, [should] act to alleviate the suffering,” the Pope said.

Meanwhile, reports emerged Sunday that ISIS released at least 19 of the Assyrian Christians it kidnapped.

Those freed were 50 years of age or older; 16 were men and three women, according to AP.

The group had all been from the same village and they arrived Sunday safely at the Church of the Virgin Mary in the city of Hassakeh after traveling by bus from the ISIS-held town of Shaddadeh.

The captives had been ordered freed by a Shariah court after paying an unspecified amount of money, which was levied as a tax on non-Muslims, the Assyrian Human Rights Network said.

ISIS militants first began rounding up scores of civilians — most of them women and children — Feb. 23 after overrunning a string of villages in northeast Syria, according to the Britain-based activist group Syrian Observatory for Human Rights.

Another US military official, also speaking anonymously, described the range of possible scenarios that could unfold now that ISIS is rounding up captives by the hundreds.

“One is enslavement, which consists of cooking,cleaning and digging holes for [ISIS] soldiers on the front lines,” the official said.

“And then of course, there’s the scenario involving executions. “That’s our greatest fear.”​