The US president, who met with Putin several times on the margins of the just-concluded APEC summit in the Vietnamese resort of Danang, took a fresh swipe at critics of his efforts to forge a close working relationship with the Russian leader.

When will all the haters and fools out there realize that having a good relationship with Russia is a good thing, not a bad thing. There always playing politics - bad for our country. I want to solve North Korea, Syria, Ukraine, terrorism, and Russia can greatly help! — Donald J. Trump (@realDonaldTrump) November 12, 2017

Does the Fake News Media remember when Crooked Hillary Clinton, as Secretary of State, was begging Russia to be our friend with the misspelled reset button? Obama tried also, but he had zero chemistry with Putin. — Donald J. Trump (@realDonaldTrump) November 12, 2017

"When will all the haters and fools out there realise that having a good relationship with Russia is a good thing, not a bad thing," he tweeted.

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

Speaking to reporters on Air Force One while flying to Hanoi on Saturday, Trump said he believed Vladimir Putin was being sincere when he denied meddling in the 2016 US presidential election.

Trump, whose key former aides are under US investigation for possible collaboration with the Kremlin, said he had repeatedly asked Putin about the claims during their chats in Danang.

"He (Putin) said he didn't meddle. I asked him again," Trump, who is marking one year since his shock election victory, told the reporters.

"I really believe that when he tells me that, he means it," he added.

Trump's Sunday morning tweets also focused on North Korea and its nuclear weapons ambitions that have been a dominant theme on each leg of his Asia tour.

President Xi of China has stated that he is upping the sanctions against #NoKo. Said he wants them to denuclearize. Progress is being made. — Donald J. Trump (@realDonaldTrump) November 11, 2017

'Short and fat'

Taking exception to descriptions by North Korean officials and state media of him as an "old" man, Trump declared himself disappointed by what he took to be 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 said.

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

Why would Kim Jong-un insult me by calling me "old," when I would NEVER call him "short and fat?" Oh well, I try so hard to be his friend - and maybe someday that will happen! — Donald J. Trump (@realDonaldTrump) November 12, 2017

North Korea is extremely sensitive to any remarks -- even if not meant seriously -- that it sees as disrespectful of the country' 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.

Over the past week, Trump has urged Asian leaders to take a united front against the threat posed by the isolated North, warning at APEC that the region "must not be held hostage to a dictator's twisted fantasies".

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

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 China.

"President Xi of China has stated that he is upping the sanctions against (North Korea). Said he wants them to denuclearise. 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.

China has made no sanctions announcement in recent days, and it was unclear if Trump was referring to statements Xi may have made during their summit in Beijing on Thursday, or when they met at APEC.