How should the United States respond to Turkey's downing of a Russian fighter jet on Tuesday? Candidates for the Republican Party presidential primaries are divided on this contentious issue.

Florida Senator Marco Rubio said the U.S. should be ready to defend Turkey, its ally in the North Atlantic Treaty Organisation (NATO), against Russian aggression.

He said the latest incident involving Turkey and Russia will be a test for NATO as an alliance.

"If Russia believes that they can respond and retaliate against Turkey because NATO's not going to do anything about it, what they are basically doing is they are making the argument that NATO is no longer viable, that it is a feckless alliance," Rubio told Fox News on Tuesday, as quoted by CNN.

"So it's important for us to be clear that we will respond and defend Turkey if they come under assault from the Russians," he added.

Rubio, who has been calling for greater U.S. military spending, said the downing of the Russian fighter jet attacking targets in Syria illustrates why America should be wary of Russia's presence in the Middle East.

"It's the reason why I warned about having Russia in the Middle East," he said. "I warned exactly, specifically, this could happen. That there could be an incident with Turkey, a NATO ally. That they could aggressively fly over their territory and be shot down."

On Tuesday, Turkey shot down the Russian warplane carrying two crew members, saying it was encroaching on its airspace.

In response to the incident, Russian President Vladimir Putin warned Turkey of "significant consequences," insisting that the downed warplane "posed no threat."

Kentucky Senator Rand Paul, another Republican presidential hopeful, meanwhile took a softer stance on Russia amid tensions with Turkey, saying the U.S. should continue to dialogue with its former rival during the Cold War.

"The shooting down of a Russian fighter jet illustrates precisely why we need open lines of communication with Russia and should resist calls from some presidential candidates to isolate ourselves from discussions with our adversaries," Paul said.