The full interview airs on CNN Tonight at 10 p.m. ET
"They don't respect our president. They really don't respect us anymore. And that's why they're doing this," Trump told CNN's Don Lemon in a wide-ranging interview at Trump Tower Wednesday. "At the same time, if they want to hit ISIS, that's OK with me."
Trump's remarks came in response to the latest escalation of tensions in the Middle East, where Russia launched its first airstrikes in Syria
. Moscow claims the target is ISIS.
However, top U.S. officials have raised questions about the real motivations behind the strikes. U.S. Defense Secretary Ash Carter said it appeared they were "in areas where there probably were not ISIL forces."
"The result of this kind of action will inevitably, simply be to inflame the civil war in Syria," Carter said.
Trump told CNN Wednesday that he tends to believe Russia's goal is to go after ISIS and that the U.S. shouldn't strive to be the "policemen of the world."
"I hear they are hitting both," said Trump, apparently referring to ISIS and non-ISIS Syrian opposition forces.
"If Russia wants to go in and if Russia want to fight -- in particular ISIS, and they do and one of the reasons they do is because they don't want ISIS coming into their country and that's going to be the next step. So that's why they're there," Trump said. "I think they will be fighting ISIS."
He called Syrian President Bashar al-Assad a "bad guy" who has killed hundreds of thousands of people, and that Russia is "probably trying to prop up Assad and help him out."
"We always give weapons, we give billions of dollars in weapons and then they turn them against us. We have no control. So we don't know the other people that we're supposed to be backing," Trump said of U.S. involvement in the region. "We don't even know who we are backing."
The turmoil on the other side of the world has triggered a refugee crisis, with thousands fleeing violence in countries like Syria and Iraq.
Obama said the United States will accept at least 10,000 Syrian refugees
into the country next year -- a number that some Democrats and humanitarian groups have said it much too low. The Obama administration recently said
the total number will be increased to 85,000 in 2016 and 100,000 in 2017.
Many Republicans, including Trump, are critical of the decision to accept refugees from the region at all, arguing the process could be exploited by terrorists attempting to travel to the U.S. to do harm.
Trump on Wednesday went one step beyond that. He told CNN the U.S. shouldn't accept people that "may be ISIS," and vowed that as president, he would expel the refugees let in by the Obama administration.
"We have no idea who they are. We have no idea where they come from. And I'm just telling you right now, they may come in, through the weakness of Obama, but they're going out if I become president," Trump said. "They will not stay here. They are going back to Syria, whether it's safe zones or whatever."
In his first months as a presidential candidate, Trump has discussed foreign policy in largely sweeping terms. The campaign has yet to release anything formal on how Trump would handle national security issues or what kind of commander-in-chief he would strive to be.
Pressed repeatedly about his lack of foreign policy proposals, Trump said it's important to be "unpredictable."
"I don't like talking so specific -- I'm going to do this, I'm going to do that, like a fool," he said. "I want to be unpredictable."