“When you get these terrorists, you have to take out their families,” Trump said on Fox News.
“They care about their lives, don’t kid yourselves.”
Trump said if he were president, he would try to avoid civilian deaths in going after the militant group, but he said the Obama administration was “fighting a very politically correct war”.
Christoph Wilcke of Human Rights Watch said in response to Trump’s comments that military forces legally can only target combatants.
“The family members of fighters are civilians and cannot be targeted,” he said.
Trump’s comments about the families of IS fighters came a day after Lebanon released the ex-wife of the group’s leader, Abu Bakr al-Baghdadi, and other jailed Islamists in an exchange with al-Qaida’s Syrian wing.
Saja al-Dulaimi told al-Jazeera after her release she wanted to return to Beirut and then leave for Turkey.
Trump leads the pack seeking the Republican nomination for the November 2016 presidential election.
In a recent Reuters/Ipsos poll of the Republican field, the real estate mogul received support of 29%.
“I would knock the hell out of ISIS,” Trump said.