US Republican presidential nominee Donald Trump said he will "respectfully" ask South Korea and other allies to pay more for American defense support if elected to the White House.

Trump made the remark during a campaign speech in Philadelphia, once again reaffirming one of the key points of his foreign policy: that allies take US defense support for granted and don't pay their fair shares.

"I will be respectfully asking countries such as Germany, Japan, South Korea, Saudi Arabia to pay more for the tremendous security we provide them," Trump said.

Trump also said that only five NATO countries, including the US, are meeting their minimum requirement to spend 2 percent of their gross domestic product on defense.

"They know they have to do it. They can afford to do it. They have no respect for our leadership. They have no respect for our country. They will do it. They'll be happy to do it. They will be happy to do it," he said.

The real-estate tycoon has expressed deeply negative views of alliances and US security commitments overseas, seeing them as a cumbersome burden sucking up taxpayer dollars, and arguing that the US should no longer be the "policeman of the world."

He has said allies should pay 100 percent of the cost of stationing American troops, or the US should be prepared to end their protection. He even suggested allowing South Korea and Japan to develop their own nuclear weapons for self-defense so as to reduce US 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, which ended in a truce, not a peace treaty. Many agree that the troop presence is also in line with US interests in a region marked by China's rise. (Yonhap)