Faced with increasing scrutiny about whether Mr. Carson — who leads in some Republican presidential polls — was capable of leading American foreign policy, two of his top advisers said in interviews with The New York Times that he had struggled to master the intricacies of the Middle East and national security and that intense tutoring was having little effect.

“Nobody has been able to sit down with him and have him get one iota of intelligent information about the Middle East,” Duane R. Clarridge, a top adviser to Mr. Carson on terrorism and national security, said in an interview. He also said Mr. Carson needed weekly conference calls briefing him on foreign policy so “we can make him smart.”…

This week, Mr. Carson’s advisers said that his source for claiming Chinese involvement in Syria was a telephone conversation he had had with a freelance American intelligence operative in Iraq…

Mr. Clarridge, a former C.I.A. agent who had connected Mr. Carson with the operative in Iraq, said on Monday that the information was wrong. The operative in Iraq had “overleaped” in suggesting Chinese troops are in Syria, Mr. Clarridge said, adding of the operative, “You know how it goes when people are desperate for some headline.”