Steven Pifer, a former U.S. ambassador to Ukraine, told the news service that if the two meet, Trump has to “say to Putin, 'We’re not happy about you interfering in our election.’”
“If you don’t say that, you are going to get hammered by the press and Congress and you can guarantee Congress will pass sanctions legislation against Russia," he said.
“They also need to keep their expectations very, very modest,” Pifer added. “If they aim for a home run in Hamburg, my guess is they’ll strike out.”
The meeting would come amid multiple investigations into the Russia's meddling in the U.S. presidential race, including into any possible collusion between members of Trump's campaign and Moscow.
Trump has decried the Russia probe as a "witch hunt," and Putin has denied interference in the U.S. election.