President Donald Trump said on Friday he considered Japanese Prime Minister Shinzo Abe a great friend and expressed concern over his decision to step down for health reasons.

"I want to pay my highest respect to Prime Minister Shinzo Abe ... a great friend of mine," Trump told reporters on Air Force One. "I just feel very badly about it."

Trump said Abe loved his country very much and said he planned to call him.

Abe announced on Friday he was resigning because of poor health, his long-running battle with ulcerative colitis ending a tenure as Japan's longest-serving prime minister.

Abe's team is arranging the call for Monday Japan time, with the prime minister informing the president directly about his resignation, Kyodo news agency said, citing unnamed government sources.

A Foreign Ministry official declined to comment.

Abe, the first foreign leader to meet with Trump as president-elect, forged close ties with him, playing golf and engaging in frequent phone calls and meetings.

He said in his resignation announcement that strengthening the U.S.-Japan alliance helps boost Japan's deterrence.