U.S. President Donald Trump said Friday he pays his "highest respect" to outgoing Prime Minister Shinzo Abe, expressing sympathy that "a great friend" had to decide to step down due to health problems.

"We've had a great relationship and I just feel very badly about it, because it must be very severe for him to leave," Trump told reporters aboard Air Force One, adding, "He loves his country so much and for him to leave, you know, I just can't imagine what it is."

Abe plans to talk by phone with Trump on Monday and call for bilateral cooperation to continue under his successor's upcoming team, Japanese and U.S. government sources said Saturday.