Donald Trump says he again asked Vladimir Putin if he interfered in the US election and the Russian leader reassured him "he absolutely did not meddle".
"I really believe that when he tells me that, he means it," Mr Trump told reporters on Air Force One.
He said every time he sees the Russia leader, Mr Putin tells him: "I didn't do that" and is "very insulted" by claims he interfered.
"You can only ask so many times… He said he absolutely did not meddle in our election," said the President.
Mr Trump said the pair had talked informally "two or three" times in the last few days on the sidelines of the Asia Pacific Economic Cooperation (APEC) conference in Vietnam.
US intelligence agencies believe Russia interfered in last year's presidential election to help Mr Trump get into the White House.
Mr Trump's former campaign manager, Paul Manafort, is one of those implicated and has pleaded not guilty to charges including conspiracy against the US.
But Mr Putin told reporters in Vietnam that Manafort's alleged links with Russia had been fabricated by the US President's opponents.
He also said allegations that a woman described as his niece had met Mr Trump's former adviser George Papadopoulos were "some kind of fantasies."
"Everything about the so-called Russian dossier in the US is a manifestation of continuing domestic political struggle," said Mr Putin.
There had been mixed messages on whether the two leaders would meet at the Vietnam summit.
Despite no formal sit-down meeting – apparently because of scheduling clashes and protocol – the pair spoke informally and had time to talk about Syria.
The US President was seen giving his Russian counterpart a friendly slap on the shoulderas the two men – in matching blue shirts – stood next to each other for a photo ahead of Friday's dinner.
A joint Kremlin and White House statement on Saturday said they had reaffirmed their intention to defeat Islamic State in Syria.
It said they agreed to support existing communication channels to ensure their troops' safety in the country, and to continue efforts until the terror group is defeated.
They also agreed that the Syrian conflict "does not have a military solution" and must be resolved through talks, the so-called "Geneva process".
Mr Trump is now in Vietnamese capital Hanoi as he comes to the end of his Asia tour.
North Korea said on Saturday that Mr Trump had "begged for a nuclear war" during his trip.
"His current trip to our surrounding region is a warmonger's visit for confrontation to rid the DPRK of its self-defensive nuclear deterrence," said a foreign ministry spokesman.
The President has "laid bare his true nature as destroyer of world peace and stability and begged for a nuclear war on the Korean peninsula", the statement added.
"A dotard like Trump can never frighten us or put a stop to our advance," said the spokesman, whose statement was released via the North's KCNA news agency.
The secretive regime has carried out a number of rocket tests this year as Mr Trump and Kim Jong Un trade threats.
The President earlier this week warned Pyongyang not to underestimate the US and called on China – North Korea's biggest trading partner – and Russia to do more.
Mr Trump, speaking at a business event in Beijing, said: "Time is quickly running out.
"We must act fast and hopefully China will act faster and more effectively on this problem than anyone.
"I'm also calling on Russia to help rein in this this potentially very tragic situation."
While in South Korea, Mr Trump also offered North Koreans a "path towards a much better future", urging the regime to "come to the table" and "make a deal" over its nuclear programme.