William Hagerty, the new U.S. Ambassador to Japan, vowed Thursday to maintain "ironclad" bilateral security ties in the face of North Korea's escalating nuclear and missile threats and said he expects Tokyo to assume a greater role in the decades-old alliance Washington regards as a cornerstone of regional stability.

"I expect to see us strengthening our position in the face of the increasing threat here in this region, and I also expect to see further involvement of Japan and further responsibility and authority of Japan," he said, speaking to reporters after arriving at Narita airport near Tokyo in late afternoon.

"Our partnership with Japan is ironclad and our ability to defend ourselves becomes even stronger as we work more closely together," he said.