GOP presidential nominee Donald Trump is not the only person worried deteriorating relations with Russia could lead to war. 

Director of National Intelligence James Clapper said Tuesday that a no-fly zone in Syria, a proposal heavily supported by Democratic nominee Hillary Clinton, could lead Russia to shoot down U.S. planes. 

"I wouldn’t put it past them to shoot down an American aircraft if they felt that was threatening to their forces on the ground," Clapper said at the Council of Foreign Relations. "The system they have there is very advanced, very capable, and I don’t think they’d do it – deploy it – if they didn’t have some intention to use it."

Clinton says that, as president, she would help implement a no-fly zone over Syria to protect civilians from military strikes. The multifaceted war in the country, stemming from a rebellion against Syrian President Bashar Assad's rule, has entered its sixth year, leaving thousands dead and sparking a massive refugee crisis. 

But Clinton's plan is a departure from top U.S. officials, including President Barack Obama, who would prefer to focus on persuading Assad to step down. They say a no-fly zone would put the U.S. directly in conflict with Russia, who is currently working with the Syrian government in eliminating ISIS, other terrorist groups and rebel groups in the country.

Trump has agreed, adding that the move could trigger a world war

"What we should do is focus on ISIS. We should not be focusing on Syria. You’re going to end up in World War Three over Syria if we listen to Hillary Clinton," Trump told Reuters Tuesday. "You’re not fighting Syria anymore, you’re fighting Syria, Russia and Iran, all right? Russia is a nuclear country, but a country where the nukes work as opposed to other countries that talk."

The issue is further complicated by reports from human rights organizations, like the Syrian Observatory for Human Rights, which say Russian and Syrian airstrikes have continued to plague Aleppo and other conflict areas, leaving civilian casualties. The U.N has denounced such airstrikes as war crimes. 

During the third presidential debate last week, moderator Chris Wallace asked Clinton whether she would fire on Russian aircraft that were violating a potential U.S.-imposed no-fly zone. She said she thought the no-fly zone could be imposed peacefully.

"I think a no-fly zone could save lives and hasten the end of the conflict," Clinton responded. "I am well aware of the really legitimate concerns you have expressed from both the president and the general. This would not be done just on the first day. This would take a lot of negotiation and it would also take making it clear to the Russians and the Syrians that our purpose is to provide safe zones on the ground. We've had millions of people leave Syria, and those millions of people inside Syria who have been dislocated. So I think we could strike a deal and make it very clear to the Russians and Syrians that this was something that we believe the best interests of the people on the ground in Syria. It would help us in the fight against ISIS."