U.S. Republican presidential front-runner Donald Trump said Wednesday that American allies should pay more for U.S. defense support or defend themselves on their own.

Trump made the remark during a major foreign policy speech, a day after he won all of the five primaries at stake on Tuesday and moved a step closer to clinching the nomination.

The real-estate tycoon has long made the case for making allies shoulder more of the costs necessary for American troop presence and defense support, but Wednesday's remark was seen as carrying more weight than before because it was the first time Trump has outlined his foreign policy priorities in such a formal and refined manner.

In Wednesday's address, Trump laid out "five main weaknesses in our foreign policy" and the cost-sharing with allies was the No. 2 problem. Though Trump made no mention of South Korea, the remark suggests that if elected, he could pursue U.S. troop withdrawal from the Asian ally unless Seoul agrees to pay more.

"Our allies are not paying their fair share, and I've been talking about this recently a lot. Our allies must contribute toward their financial, political, and human costs, have to do it, of our tremendous security burden. But many of them are simply not doing so," Trump said during the speech at a Washington hotel.

"We have spent trillions of dollars over time on planes, missiles, ships, equipment, building up our military to provide a strong defense for Europe and Asia. The countries we are defending must pay for the cost of this defense, and if not, the U.S. must be prepared to let these countries defend themselves. We have no choice," he said.

If elected president, Trump said he would call for "a summit with our NATO allies and a separate summit with our Asian allies" to "discuss a rebalancing of financial commitments but take a fresh look at how we can adopt new strategies for tackling our common challenges."

On North Korea, Trump said U.S. President Barack Obama watched "helplessly" as Pyongyang "increases its aggression and expands further and further with its nuclear reach." He said the communist nation is now "totally out of control."

He again stressed that the U.S. should make China use more of its influence with Pyongyang.

"Our president has allowed China to continue its economic assault on American jobs and wealth, refusing to enforce trade deals and apply leverage on China necessary to rein in North Korea," Trump said. 

"We have the leverage. We have the power over China, economic power, and people don't understand it. And with that economic power, we can rein in and we can get them to do what they have to do with North Korea, which is totally out of control," he said.

Trump has long made unfounded claims that the U.S. has been protecting wealthy nations like South Korea and Japan in exchange for little and should end such protection unless those countries agree to pay more. He has even suggested allowing the allies to arm themselves with nuclear weapons so as to reduce U.S. security burdens.

About 28,500 American troops are stationed in South Korea to deter North Korean aggression, a legacy of the 1950-53 Korean War that ended in a truce, not a peace treaty, leaving the divided peninsula still technically at war.

Seoul has long shared the cost of stationing U.S. forces. 

In 2014, the two countries renewed their cost-sharing agreement, known as the Special Measures Agreement, with Seoul agreeing to pay 920 billion won for the upkeep of the U.S. troops in 2014, a 5.8 percent increase from a year earlier.

Moreover, the American military presence on the peninsula is seen as in line with U.S. national interests in a region marked by a rising China.

Earlier this month, Gen. Vincent Brooks, who has been nominated to be commander of U.S. Forces Korea, said that Seoul is shouldering a "significant load" of the costs for the stationing of American troops in the country.

Brooks also said that South Korea is funding most of the cost for a massive $10.8 billion project to relocate American troops farther south, which he called the "largest DOD construction project we have anywhere in the world."

He also said it would cost more to keep those troops stationed in the U.S. than it does in Korea. (Yonhap)