Trump pays 'highest respect' to resigning Japanese PM Abe

US President Donald Trump on Friday paid his "highest respect" to Japanese Prime Minister Shinzo Abe and voiced concern over his "great friend" resigning for health reasons.

"I want to pay my highest respect to Prime Minister Shinzo Abe, a very great friend of mine," Trump told reporters aboard Air Force One as he returned from a campaign rally in New Hampshire. 

"We've had a great relationship and I just feel very badly about it, because it must be very severe for him to leave."

"He loves his country so much and for him to leave, you know, I just can't imagine what it is. He's a great gentleman and so I'm just paying my highest respect," Trump added.

Abe announced earlier he was ending his record-breaking tenure, kicking off a leadership race in the world's third-largest economy.

He said he was suffering a recurrence of the ulcerative colitis that forced him to cut short a first term in office, and that he no longer felt able to continue as prime minister.

The two leaders have met several times during the US president's term, and staffers have hailed the "unprecedented" relationship between Trump and his "golf buddy."

A Japanese diplomat said last year the frequency of contact demonstrated the "unprecedented level of close personal relations" between the pair.

Trump announced in September last year that the two allies had taken a major step towards sealing a comprehensive new trade deal, after a year of negotiations between the global economic powers.

Abe was forced to leave office just one year after becoming the country's youngest-ever prime minister but has since become Japan's longest-serving premier.

Speculation about his political future had intensified after two recent hospital visits for unspecified health checks, but the resignation was nonetheless a surprise.

He had been expected to stay in office until the end of his term as LDP leader in September 2021.

Even as recently as Friday morning, the government spokesman had appeared to dismiss concerns about Abe's health and suggested he would stay on.