TOKYO (AFP) - To bow or not to bow: That was the question when US President Donald Trump held a closely watched meet-and-greet with the Japanese emperor laden with potential protocol pitfalls.

Trump's predecessor Barack Obama sparked a firestorm of criticism during one trip to Japan when he was photographed in an almost 90-degree bow to the diminutive Emperor Akihito.

US conservatives chided Obama for the act of deference towards Akihito, the son of wartime emperor Hirohito, in whose name Japanese troops fought in World War II.

So all eyes were on the meeting between the two heads of state in famously polite Japan where bowing is a way of life.

But the gaffe-prone Trump appeared to plot a respectable middle ground, slightly inclining his head upon meeting the 83-year-old despite towering over him.

The two couples were then led to a room where they engaged in deep conversation with the aid of interpreters.

Trump aides have been nervous about keeping the notorious ad-lib president on message during his gruelling Asia tour - the longest by a US leader since 1991.

And Trump is the second unorthodox world leader to meet the emperor in the space of a week.

Last Monday, Philippine President Rodrigo Duterte, not generally known for his respect of formal protocols, also received an audience at the imperial palace.

The occasionally foul-mouthed Duterte pledged to "limit my mouth" before his trip to Tokyo and Japanese media reported that he was on his best behaviour.

Japanese tabloid Yukan Fuji said the meeting was peaceful and Duterte bowed repeatedly towards the emperor and the empress afterwards.