"Do some outreach. Get to know some of these Democratic leaders," he said. "You can find common ground." McCain said he'd met Trump "some years ago" when he was a businessman, but had not met him since. McCain said he did speak "almost daily" to National Security Adviser Lt. Gen. H.R. McMaster and Defense Secretary Jim Mattis, however.

"He doesn't seem to be that upset that he's not talking to him," said German Marshall Fund's Derek Chollet, a former Obama Pentagon official. "He's trying to run U.S. defense policy through Mattis and effectively ignore Trump."

The White House did not immediately respond to requests for comment.

The senior Republican and vociferous Trump critic said Trump's tweet storms had caused "a great deal of uncertainty" about who advises the president. "Who drives the tweets at 6am in the morning?" he said, speaking at the German Marshall Fund's Brussels Forum.