Then-candidate Donald Trump speaks at the Republican presidential debate, hosted by CNN, at the Venetian in Las Vegas on Dec. 15, 2015. (Robyn Beck/AFP/Getty Images)

Back when he was still a candidate and not president of the United States, Donald Trump had a suggestion for winning the battle against the Islamic State: “Take out their families.”

“The other thing with the terrorists is you have to take out their families, when you get these terrorists, you have to take out their families,” Trump said during an appearance on Fox News' “Fox and Friends” in December 2015. “They care about their lives, don't kid yourself. When they say they don't care about their lives, you have to take out their families.”

The comment prompted criticism from other candidates, who argued that such a tactic would constitute a war crime.

A year and a half later, Trump is now president — and on Friday, a monitoring group said that airstrikes from a U.S.-backed coalition on a town in Syria had killed a large number of relatives of Islamic State fighters.

The Syrian Observatory for Human Rights, an organization based in Britain that tracks the Syrian war, said that two rounds of coalition airstrikes had hit the town of al-Mayadin in Deir al-Zour province Thursday and Friday. According to their sources, the organization said, one of the strikes had hit a building housing the families of Islamic State fighters.

In total, at least 106 people died in the strikes, the Observatory said, including 42 children of Islamic State fighters. The article announcing the deaths dubbed it “the largest massacre against the [the Islamic State's] families in Syria.”

On Friday, a Pentagon spokesman, Capt. Jeff Davis, confirmed to The Washington Post that coalition forces conducted airstrikes in Deir al-Zour province on May 25 and 26. The results of those airstrikes were still being assessed, Davis said, but allegations of civilian casualties were being taken “seriously.”

As with much in the Syrian war, finding out exactly what happened in Mayadin on Thursday and Friday is difficult. Crucially, it is not clear whether the reports that Islamic State fighters' families were killed is accurate, or whether the building purportedly housing them was the intended target of the strike.

However, multiple legal experts said that if the families of Islamic State fighters were indeed being targeted deliberately like Trump said he would, it would violate the Geneva Conventions. “Family members of combatants would be considered civilians,” David Bosco, an associate professor at Indiana University and author of a book on international law, said. “Targeting them intentionally would be illegal.”

Steven R. Ratner, a professor at University of Michigan Law School, agreed, adding that such an attack would be a “grave breach” of the Fourth Geneva Conventions, which protects civilians during times of war. Even if any civilian deaths were accidental, Ratner said that “an attack that is not intentionally targeted at civilians but is done with the knowledge that it will cause disproportionate harm to civilians compared to the military advantage gained is also a war crime.” Ratner said this was described in Protocol I of the Geneva Conventions, which the United States is not a party to but generally accepts.

However, even if the strike on Mayadin did intentionally target civilians, there's little immediate chance of an international trial. “There's no international forum that would have jurisdiction to try these things,” Bosco said. The International Criminal Court does not have jurisdiction in Syria nor the United States, making any potential trial a “distant hypothetical,” according to Bosco.

Ratner suggested that it was possible that there could be a court-martial under the Uniform Code of Military Justice, which allows for trials of war crime, but said that was unlikely unless there was clear evidence of a deliberate attack on civilians. Sarah Knuckey, director of Columbia Law School's Human Rights Clinic, also noted that under universal jurisdiction, foreign states could permit their national courts to try Americans as war criminals.

Civilian casualties from the U.S.-led fight against the Islamic State in Syria and Iraq remain a touchy subject. On Thursday, the Pentagon acknowledged that a U.S. strike on a building in the Iraqi city of Mosul had killed more than 100 civilians, but blamed the high death toll on secondary explosions from weapons stored in the area by Islamic State militants. In total, the coalition has admitted to killing 352 civilians since 2014.

Monitoring group Airwars recently suggested that protection for civilians in Syria and Iraq appeared to have been “scaled back” since Trump entered office. The result, Airwars director Chris Woods said in a statement, was “the inevitable consequence of higher deaths and injuries.” However, limited access to the areas where strikes hit makes it hard to fully assess civilian casualties.

“While the United States has taken some steps to increase transparency around its responsibility for civilian casualties abroad, many gaps remain,” Knuckey said in an email. “The government's minimal public explanations often raise more questions than they answer.”

Days after Trump's comments last December on “Fox and Friends,” the candidate seemed to confirm his threat to target terrorists' families in a Republican debate hosted by CNN in Las Vegas. “They may not care much about their lives, but they do care, believe it or not, about their families’ lives,” Trump said at the December event when asked about his earlier comments.

Rand Paul, a rival candidate also at the debate, was among those who criticized Trump's suggestion. “If you are going to kill the families of terrorists,” Paul said during the CNN debate, “realize that there's something called the Geneva Convention we're going to have to pull out of. It would defy every norm that is America.”

It was only months later, during a CNN interview that aired in March 2016, that Trump said he would not kill terrorists' families but only “go after them.” However, in the same interview, the now-president suggested he did not think much of the international rules that sought to prevent governments from targeting civilians.

“It’s very interesting what happens with the Geneva Convention,” Trump said. “Everybody believes in the Geneva Conventions until they start losing, and then it’s okay, let’s take out the bomb.”

More on WorldViews

Trump once denounced Saudi Arabia as extremist. Now he’s heading there to promote moderate Islam.