"Any deliberate attacks aimed against civilians is a war crime, regardless if they are family members of terrorists or presidents or presidential candidates," said Boaz Ganor, a former consultant to Israeli Prime Minister Benjamin Netanyahu.
"Adopting this policy is immoral and against the common liberal democratic values," added Ganor, the founder and executive director of the International Institute for Counter-Terrorism in Israel. "Deliberate attacks against the terrorist families is blurring the moral differences between the terrorist organizations and the state which is fighting terrorism. This by itself might benefit the terrorists which are trying to claim that they are fighting a moral war against relentless and immoral entity."
On Wednesday, Trump responded to a question on Fox News about minimizing civilian casualties in the fight against ISIS by saying the U.S. should hit the kin of enemy fighters.
"You have to take out their families, when you get these terrorists, you have to take out their families," Trump said. "When they say they don't care about their lives, you have to take out their families."
Ganor, who worked as part of an international anti-terror advisory group to the NYPD after 9/11 and has written extensively about terrorism, told CNN that killing the family members of terrorists also would have little benefit in the effort to stop attacks.
"They might just spend more time and resources" on protecting family members, he said. "Nevertheless, I don't think that the threat to kill their families will deter terrorist leaders form being engaged in terrorist activities."
Ganor argued that the real danger would be in squandering "one of the most important pillars of counterterrorism: the differences of morality."
"The American-led campaign in Syria and Iran is too little and too late, and it will not lead as such to a military decisive victory on ISIS," he said. "This does not mean by any way that the U.S. should adopt savage policy, illegal or immoral military activity. There is a lot that still can be done and improve without adopting such policies."
Asked about Israel's anti-terror efforts, which at times have caused civilian casualties in the West Bank and Gaza and resulted in the country being accused by critics such as Amnesty International of war crimes and human rights violations against Palestinian civilians, Ganor said that family members were not the targets.
"Israel activities were always calculated and limited to the immediate goals of thwarting terrorism and the measures that Israel was using in its counter terrorism campaigns where never random nor aimed against civilians even they had family ties with terrorists and their leaders," he said. "The families were not targeted nor even being arrested unless they took part in the terrorist activity, then they would brought to court and be accused in terrorism assistance."
The Trump campaign did not respond to a CNN request for comment.