Prime Minister Shinzo Abe has so far convinced the United States that he can exercise strong leadership to reshape Japan after a decade of political turmoil, but he should also be careful not to damage relations with South Korea, now strained by diplomatic tensions, a former U.S. ambassador to Japan said.

In a recent interview with The Japan Times, Tom Schieffer, who served as the top American diplomat to Japan during Abe's first stint as prime minister up until 2007, said Abe's announcement earlier this month that Japan would join the Trans-Pacific Partnership trade talks helped persuade the U.S. that he knows where he wants to take Japan.

"Abe certainly was disappointed when he left the prime ministership for the first time. Sometimes when you reflect on those things, it makes you a stronger person," said Schieffer, who met with Abe during his visit to Washington last month. "He looked to me as if he was enjoying being a prime minister more than the first time."