Donald J. Trump, the Republican presidential front-runner, said that if elected, he might halt purchases of oil from Saudi Arabia and other Arab allies unless they commit ground troops to the fight against the Islamic State or “substantially reimburse” the United States for combating the militant group, which threatens their stability.

“If Saudi Arabia was without the cloak of American protection,” Mr. Trump said during a 100-minute interview on foreign policy, spread over two phone calls on Friday, “I don’t think it would be around.”

He also said he would be open to allowing Japan and South Korea to build their own nuclear arsenals rather than depend on the American nuclear umbrella for their protection against North Korea and China. If the United States “keeps on its path, its current path of weakness, they’re going to want to have that anyway, with or without me discussing it,” Mr. Trump said.

And he said he would be willing to withdraw United States forces from both Japan and South Korea if they did not substantially increase their contributions to the costs of housing and feeding those troops. “Not happily, but the answer is yes,” he said.