Manila (Philippines) (AFP) - Donald Trump said Sunday he backed the US intelligence agencies which concluded that Russia meddled in the 2016 US presidential election, but slammed "fools" who oppose better ties with Moscow.

Key former Trump aides are under US investigation for possible collaboration with the Kremlin, and the issue of whether Moscow interfered with last year's vote has overshadowed the tail end of the president's 12-day Asia tour.

Trump returned to the subject in a Twitter storm in Vietnam, which also saw him take a sarcastic dig at North Korea's "short and fat" leader Kim Jong-Un.

At a press conference in Hanoi, Trump was asked to clarify comments he had made the day before about Russian President Vladimir Putin's insistence that Moscow had never tried to affect the US vote.

"I believe he feels he and Russia did not meddle in the election," Trump said.

"As to whether or not I believe it or not, I'm with our agencies. I believe in our... intelligence agencies," he added.

But in his barrage of tweets, Trump slammed "haters and fools" who questioned his efforts to improve ties with Russia.

"There (sic) always playing politics - bad for our country. I want to solve North Korea, Syria, Ukraine, terrorism, and Russia can greatly help!" he said.

In May, US intelligence chiefs told Congress they agreed with their analysts' conclusion that Russia had interfered in the election, and on Saturday, CIA director Mike Pompeo, a Trump appointee, said he still held to that evaluation.

But after sticking to script through most of his high-profile Asia trip, Trump lashed out at three former US officials whose agencies helped produce the original intelligence finding of Russian interference. He called former CIA chief John Brennan, former director of national intelligence James Clapper and former FBI chief James Comey "political hacks."

Appearing later on CNN, Brennan denounced Trump's attack as "reprehensible," and Clapper said that for Trump to so minimize a Russian attempt to undermine the US system was "astounding and, in fact, poses a peril to this country."

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- 'Short and fat' -

Trump's tour of Asia, which began its final leg in Manila on Sunday, has been dominated by the issue of North Korea and its ambitions to become a full-fledged nuclear state.

His public pronouncements on Pyongyang over the last week have veered from denouncing it as a "cruel dictatorship" to offering a hand of friendship to Kim.

On Sunday, his tone shifted back to one of hostility.

Citing descriptions by North Korean officials and state media of him as an "old" man, Trump suggested he was disappointed by what he took as a personal attack from the North's young leader.

"Why would Kim Jong-un insult me by calling me 'old,' when I would NEVER call him 'short and fat?'" Trump wrote.

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

He later insisted he hadn't been joking about eventually befriending a man he denounced last week as a "twisted" dictator.

"It's certainly a possibility. If that did happen, it would be a good thing," he told reporters.

North Korea is extremely sensitive to any remarks that might appear disrespectful to the country's ruling Kim dynasty, whose members are revered as near deities.

Since becoming president, Trump has engaged in an escalating war of words with Kim, trading personal insults and threats of military strikes, and raising concerns about an outbreak of hostilities.

Pyongyang hit back Saturday, calling his Asia trip "a warmonger's visit for confrontation" and saying it would only serve to accelerate Pyongyang’s nuclear program.

- South China Sea support -

In another tweet Sunday, Trump said Chinese leader Xi Jinping had agreed to toughen sanctions against North Korea, whose impoverished economy is hugely reliant on trade with its giant neighbour.

"President Xi of China has stated that he is upping the sanctions against (North Korea). Said he wants them to denuclearize. Progress is being made," he wrote.

The US administration thinks China's economic leverage over North Korea is the key to strong-arming Pyongyang into halting its nuclear weapons and missile programmes.

Trump is in the Philippines for a gathering that includes leaders of the Association of Southeast Asian Nations (ASEAN), which marked its 50th anniversary Sunday with a lavish dinner.

Several members of the group, including Hanoi and Manila, claim part of the resource-rich South China Sea, a key trade route over which Beijing says it has dominion and in which it has built militarised artificial islands.

Vietnam has sought support from Washington in the dispute, and dealmaker Trump said Sunday he could help solve the conflict.

"If I can help mediate or arbitrate, please let me know... I am a very good mediator," Trump told Vietnamese President Tran Dai Quang.

In the past, China has reacted angrily to any suggestion of US mediation, saying Washington has no role in the dispute.

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