Sen. John McCain (R-Ariz.) said from the U.S. Embassy in Kabul on Sunday that he remains concerned about Donald Trump's foreign policy acumen but is nevertheless standing by a pledge to back the presumptive Republican presidential nominee.

"I’ve said I would support the nominee of the party," McCain said in an interview on CBS's "Face the Nation." "I have strong disagreements, and we’ve just been through several of them, and that’s my position."

McCain, the 2008 GOP nominee, also criticized presumptive Democratic nominee Hillary Clinton. Asked by CBS's John Dickerson which candidate would handle complex foreign policy matters better, McCain declined to choose between Clinton and Trump, saying he has been unimpressed by both.

"I don’t think either one of them has displayed what I think is the necessary strategy and outlook — the planning and reliance on our military leaders — that will be necessary to succeed," he said. "I hope that whichever one is president that they would call in the David Petraeuses and the Robert Fords and the Ryan Crockers and those individuals, both military and State Department, and diplomats who succeeded in Iraq before the president gave it all away, who know what we need to do to defeat this threat, both militarily and diplomatically and in other ways. And either one of them should call them in and do what they recommend, and that way we can still succeed because America is still the strongest nation on Earth."

McCain was the subject of one of Trump's most memorable campaign insults. Last summer, Trump declared that the senator, a prisoner of war during the Vietnam conflict, is "not a war hero."

McCain was joined in the CBS interview by Sen. Lindsey Graham (R-S.C.), one of Trump's primary opponents and harshest critics. Graham also knocked Trump on foreign policy, citing his willingness to leave Syrian President Bashar al-Assad in power, but was more charitable toward Clinton.

"Mr. Trump, when you said Assad should stay, you need to rethink that," Graham said. "As to Secretary Clinton, she said she wants a no-fly zone in Syria. That is a great step in the right direction."