Donald Trump said he asked Vladimir Putin twice about Russian interference in the 2016 presidential election, but then changed the subject because he did not want the discussion to "end up in a fist fight".
The Russian leader has consistently denied Moscow's involvement in the hacking, despite US intelligence agencies concluding Russia was behind a series of acts.
"I said to him, were you involved with the meddling in the election? He said, absolutely not. I was not involved. He was very strong on it," Mr Trump said, recounting the G20 encounter to journalists.
"I then said to him again, in a totally different way, were you involved with the meddling. He said, I was not — absolutely not."
The US President said he then moved the discussion onto the Syrian conflict, saying: "What do you do? End up in a fist fight with somebody, okay?"
Mr Trump said it was important to "have dialogue" with foreign leaders such as Mr Putin, because "we got to solve Syria. We’ve got to solve Ukraine."
During the interview with journalists on board Air Force One, Mr Trump said he did not think Mr Putin would support him, given his commitment to build up the military and reduce US dependency on foreign oil.
"The next time I’m with Putin, I’m going to ask him: who were you really for? Because I can’t believe that he would have been for me," Mr Trump said.
He argued that Hillary Clinton would have cut investment in domestic energy sources an increased US dependence on foreign oil, saying: “That’s great for Russia.”
He denied reports he wanted to lift sanctions on Moscow, although he refused to say whether he would sign a new bill approving new trade restrictions against Russia.
"We have very heavy sanctions on Russia right now," he told journalists.
