Trump said he would "absolutely, firmly ask" Putin about the meddling at their planned meeting in Helsinki on July 16.
Asked if he considered Putin to be ruthless, Trump said: "I can't tell you that..."
Back in Washington, however, investigators are trying to discover whether Trump, his family and advisors already had a surreptitious working relationship with Russians when he ran for and won the presidency in 2016.
On Saturday, Twitter suspended two accounts allegedly used by Russian officials to spread information hacked from Democrats' computers and email accounts during the 2016 US presidential election.
But Russia's foreign ministry said the claims were a "heap of conspiracy schemes" meant to "damage the atmosphere" before Monday's summit.
"The Internet allows foreign adversaries to attack America in new and unexpected ways", Deputy Attorney General Rod J Rosenstein said.
The 29-page indictment lays out how, months before Americans went to the polls, Russians schemed to break into key Democratic email accounts, including those belonging to Clinton campaign chairman John Podesta, the Democratic National Committee and the Democratic Congressional Campaign Committee.
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A Finnish craft brewery has created a "problem-solving" beer that they hope could help Donald Trump and Vladimir Putin settle their issues "like adults". He said reports the US makes about Russian interference in elections were just "assertions" and contained "no single fact".
An indictment released Friday of 12 Russian Federation spies for hacking Democratic documents during the campaign makes clear Mueller is examining whether Trump staff and allies colluded with the leaks.
Mr. Mueller did not allege that Trump campaign associates were involved in the hacking effort, that Americans were knowingly in touch with Russian intelligence officers or that any vote tallies were altered by hacking.
President Trump is scheduled to meet with Putin in Helsinki, Finland, to round out his recent European diplomatic tour that included a North Atlantic Treaty Organisation summit in Brussels and a visit to the United Kingdom.
"Where is the DNC Server, and why didn't the Federal Bureau of Investigation take possession of it?" They also created a fake identity, Guccifer 2.0, who claimed to be a Romanian hacker with "totally no relation to the Russian government".
The indictment says the Russian operatives wrote to a unnamed person "who was in regular contact with senior members" of the Trump campaign.
In February, Mueller charged 13 Russians and three Russian companies in an elaborate conspiracy to interfere in the election.
Referring to Mueller's probe, he said: "We do have a - a political problem where - you know in the United States we have this stupidity going on".
The charges include conspiracy to commit an offense against the USA, aggravated identity theft and conspiracy to launder money.