Surging Republican presidential candidate Dr. Ben Carson once again appeared to be casting doubt on President Barack Obama's real religious leaning, saying the president would have suffered a "seizure" if the gunman who targeted Christians at a college in Oregon had singled out Muslims instead.

"He would've had a seizure over it," Carson told Steve Malzberg in an exclusive interview on Newsmax TV that aired Tuesday.

In his first reaction to the Oct. 1 shooting in Roseburg, Oregon, Obama never said anything that could be construed as a condemnation of the grisly murders. Instead, he deliberately made it a point to "politicise" the incident, saying what happened only demonstrated the need for tougher gun control legislation.

Carson said this was not the first time Obama showed his "reticence" in mentioning Christians when they are being massacred, like what is happening in the Middle East.

"I'm not sure why he has so much trouble actually articulating the words. It does make you wonder a little bit," Carson said.

In the Oregon carnage, gunman Chris Harper-Mercer was said to have asked his victims if they were Christians before gunning them down.

Carson said Obama is wrong in blaming the massacre on America's gun culture. "This is not an issue of the gun. This is an issue of the gunman," he said.

Carson believes Obama wants to confiscate all handguns owned by Americans.

"He probably would like to do that, but we're not going to let that happen," Carson said.

"The Second Amendment is so crucial ... It was [American senator and statesman] Daniel Webster who said America would never experience tyranny because the people are armed."

Turning to Democratic presidential bet Hillary Clinton, Carson said he is not worried by her pledge to impose gun control through the power of an executive order.

"I'll say it again, I really don't think Hillary is going to be the nominee," Carson said. "Because even the Democrats are smart enough to realise that that would be a disaster for them."

He said he expects Vice President Joe Biden to eventually enter the race and become the Democratic presidential nominee.

Asked about his priorities if he succeeds in winning the White House next year, Carson said he would quickly focus on defence.

"I am very concerned about the weakness of our military. Our Navy is at its smallest level since 1917, the Air Force since 1940," he said.

"We're losing personnel like crazy, our generals are retiring, veterans are killing themselves at the rate of 22 to 23 a week — we have to shore up our military. If we don't get our military right, nothing else matters," he said.

Carson is currently in second place in most Republican presidential polls behind frontrunner Donald Trump.