Trump takes dig at 'short and fat' Kim Jong-un after being called 'old'

US president Donald Trump took his verbal jousting with Kim Jong-un to a new level on Sunday, taunting the North Korean leader over his height and weight before musing over the idea of them eventually becoming friends.

While they have never met, the two leaders have form when it comes to name-calling, with the US president a more than willing match for the highly rhetorical flourishes deployed by his adversary in Pyongyang.

Mr Trump has referred to Mr Kim as a "madman" and "Rocket Man" while the 33-year-old responded by calling the 71-year-old former reality TV star a "mentally deranged dotard".

On Sunday Mr Trump got down to basics, with a sarcastic tweet prompted by recent descriptions of him in the North's state media as a "lunatic old man".

"Why would Kim Jong-in insult me by calling me 'old,' when I would NEVER call him 'short and fat'?" the president tweeted from Hanoi - the penultimate stop on a lengthy Asia tour that had, until then, appeared to have moderated his Twitter habit.

"Oh well, I try so hard to be his friend - and maybe someday that will happen!" he wrote.

Sarcastic or not, the dig about Mr Kim's weight, which has increased significantly since he came to power following the death of his father Kim Jong-il in 2011, is unlikely to go down well in Pyongyang.

The members of the ruling Kim dynasty - past and present - enjoy near god-like status in North Korea, which has demonstrated extreme sensitivity to any remark that might be seen as mocking or disrespectful of the leadership.

Foreign tourists to Pyongyang are generally obliged to pay homage - laying flowers and bowing deeply - before giant statues of Kim's father and his grandfather, founding leader Kim Il-sung, at some point during their visit.

Questioned later about his Tweet, Mr Trump insisted he had not been entirely joking about one day befriending the man he described earlier this month as a dictator with "twisted fantasies".

"That might be a strange thing to happen but it's certainly a possibility," he told reporters at a press conference in the Vietnamese capital.

"If that did happen, it would be a good thing, I can tell you, for North Korea ... I don't know that it will, but it would be very, very nice if it did," he said.

Mr Trump has played hawk and dove with the North during his sweep of five Asian countries - denouncing it as a "cruel dictatorship" while offering its leader a diplomatic way out of the crisis over Pyongyang's growing nuclear arsenal.

During his election campaign, Mr Trump said he would be willing to sit down with Mr Kim and negotiate over a "hamburger" - an offer the North Korea leader has so far chosen to ignore.

Mr Trump on Sunday also risked angering China, which he has sought as an ally in curbing North Korea's nuclear ambitions, by offering to mediate in a dispute over Beijing's claims to sovereignty in the South China Sea.

China has steadfastly opposed what it calls US meddling in the dispute over waters also claimed by several South-east Asian nations, and has baulked at the US navy's incursions into what Beijing considers its territorial waters.

The US is not a claimant to the potentially oil-rich and busy waters, but it has declared that it has a national interest in ensuring freedom of navigation and overflight and the peaceful resolution of the dispute. Several nations back an active American military presence in the region to serve as a counterweight to China's increasingly assertive actions, including the construction of seven man-made islands equipped with military installations.

"I'm a very good mediator and arbitrator," Mr Trump said at a news conference with Vietnamese President Tran Dai Quang in Vietnam's capital, Hanoi, before flying to Manila for the summit of the Association of Southeast Asian Nations.

The Philippines, the head of Asean's rotational chairmanship, said member states of the 10-nation bloc have to consult each other but thanked Mr Trump for his offer.

"He is the master of the art of the deal but, of course, the claimant countries have to answer as a group or individually ... mediation involves all the claimants and nonclaimants," Philippine foreign secretary Alan Peter Cayetano said.

Philippine president Rodrigo Duterte said Chinese president Xi Jinping, during a meeting in the Vietnamese city of Danang, where they attended the annual Asia-Pacific Economic Cooperation forum this past week, assured him of China's peaceful intentions in the strategic waterway, where Beijing, the Philippines, Vietnam and three other governments have overlapping claims.

When he raised concerns over China's increasing military capability in the South China Sea, Mr Duterte said Mr Xi replied, "No, it's nothing."

"He acknowledged that war cannot be promoted by anybody, it would only mean destruction for all of us," Mr Duterte told reporters after flying back to Manila. "He knows that if he goes to war, everything will blow up."

The Chinese leader, however, would not back down on Beijing's territorial claim, Mr Duterte said, and justified his decision not to immediately demand Chinese compliance with a ruling by a UN-linked tribunal that invalidated China's sweeping claims in the South China Sea on historical grounds.

China has dismissed that ruling as a "sham" and did not participate in the arbitration case that the Philippines filed during the administration of Mr Duterte's predecessor.

Mr Duterte took steps to thaw frosty relations with China after he won the presidency last year.

"If you go to the negotiating table and you start with the statement that I am here to claim validity of our ownership, you're wasting your time. They will not talk about it," Mr Duterte said.

The Asean summit opens on Monday under extra-tight security at a theatre and convention complex by Manila Bay. Mr Duterte will host a gala dinner for nearly 20 world leaders, including Mr Trump, Russian prime minister Dmitry Medvedev, Chinese premier Li Keqiang and Japanese prime minister Shinzo Abe.

Riot police used shields and water hoses on Sunday to push back hundreds of left-wing activists who tried to hold a protest at the US Embassy and carried placards that read "Ban Trump". There were no reports of injuries in the brief scuffle and the protesters left after burning a mock US flag.