A US presidential candidate has suggested committing war crimes may be the only way to defeat extremist terror organisation Isis.

Donald Trump, who is currently leading the Republican field on 36 per cent according to a recent CNN poll, told Fox News' Fox and Friends as president he would authorise actions to "take out" militants' families.

Intentionally killing civilians is internationally regarded as a war crime. It was formally codified in the fourth Geneva Convention in 1949.

“I would knock the hell out of them [the Isis militants],” he told viewers on the right-wing talk show, presented by Elizabeth Hasslebeck, Steve Doocy and Brian Kilmaede.

President Barack Obama “doesn’t know what he’s doing,” Mr Trump claimed, before adding he would do his “best” to defeat the militants.

“But we're fighting a very politically correct war. And the other thing is with the terrorists, you have to take out their families,” he claimed.

"They, they care about their lives. Don’t kid yourself. But they say they don’t care about their lives. You have to take out their families.”