Leading Republican presidential candidate Donald Trump says he supports Russian efforts to fight and destroy the Daesh (ISIL) terrorist network in the Middle East.

During an interview with NBC on Tuesday, Trump said the United States should support countries that are helping the Syrian government battle the Takfiri terrorist group.

"I side with the group that says if Russia wants to go and fight ISIS, you should let them as opposed to saying, we’re jealous we don't want you to do that," the billionaire businessman said.

Asked about whether Syrian President Bashar al-Assad was the source of the country's unrest, Trump said it was not clear and questioned who would replace him if he was overthrown.

"The people that want to come in and replace Assad, nobody knows who they are and they could end up being worse," he said. "We're constantly going out and siding with people and they turn out to be worse than the people who were there before."

In a separate interview with Fox News on Tuesday, the GOP presidential front-runner praised Russia's recent military buildup in Syria to fight Daesh.

"If he wants to fight ISIS, let him fight ISIS," Trump said of Russian President Vladimir Putin. "I say there's very little downside with Putin fighting ISIS," he added.

As part of a plan to fight against Daesh terrorists, Russia has been beefing up its military presence in Syria, deploying warplanes, tanks and personnel to an airfield in the port city of Latakia.

Trump is leading public opinion polls among those seeking the Republican Party's nomination to become US president in the 2016 election.