Republican US presidential frontrunner Donald Trump said he would like to "sit back" and watch as Russia continues air strikes in Syria, suggesting it could be a "trap" that could bog down Moscow.

In comments aired Sunday on ABC's "This Week," Trump said he would not establish a no-fly zone over Syria, as several other candidates, including Democratic frontrunner Hillary Clinton, have suggested.

"I think what I want to do is I want to sit back and... see what happens," Trump said, before suggesting that the Soviet Union's war in the 1980s against Afghan mujahideen rebels "destroyed" the communist bloc.

"Now they're going into Syria, there are so many traps, there are so many problems. When I heard they were going in to fight ISIS, I said, 'Great, let them,'" the billionaire real estate mogul told the show.

Russia began conducting air strikes last week in Syria, in what it said would be a prolonged bombing campaign against the Islamic State group, or ISIS, and other extremist groups.

But Western countries have said the strikes are targeting moderate rebel groups and are intended merely to bolster Syrian President Bashar al-Assad, an ally of Russia.

Asked about the ongoing migration crisis spurred by fighting in Syria and Iraq, Trump reiterated previous comments that he would send back refugees the United States has promised to take in, claiming they likely included IS fighters.

He said he "saw the migration" and observed that most of those fleeing were "strong men," -- although "This Week" host George Stephanopoulos noted that half the refugees are believed to be children.

"We don't know where they're coming from, we don't know who they are. They could be ISIS. It could be the great Trojan Horse," Trump said.

He added that the United States has "screwed up the Middle East so badly, breaking up Iraq, we have so destabilized the Middle East."

But of the migrants, he added: "If I win for president, they're going out... We are not going to take responsibility."

During a separate interview aired Sunday on NBC's "Meet the Press," Trump said that he thought things would be better for the Middle East if Assad were stronger.

He added that he believed the situation in the region would also be much improved if Moamer Kadhafi were still in power in Libya and Saddam Hussein in Iraq.

"Of course it would be" better, he told the show, adding: "Libya is not even a country anymore."

"It's not even a contest...Iraq is a disaster," Trump said.