Story highlights Democratic senators have written President Donald Trump a letter

Trump acknowledged the possibility of Russian meddling during a news conference

(CNN) Five senior Democratic senators involved in national security, foreign policy and intelligence wrote President Donald Trump in a letter published Thursday, urging him to use his upcoming bilateral meeting with Russian President Vladimir Putin to "make absolutely clear" that Russian interference in US elections "will in no way be tolerated."

"We believe it is crucial for you -- as the President of the United States -- to raise this matter with President Putin and to ensure that he hears you loud and clear -- interfering in our elections was wrong in 2016 and it will not be permitted to happen again," said the letter signed by Senate Minority Leader Chuck Schumer of New York, Assistant Democratic Leader Dick Durbin of Illinois and the top Democrats on the intelligence, armed services, and foreign relations committees: Mark Warner of Virginia, Jack Reed of Rhode Island and Ben Cardin of Maryland respectively. "The upcoming elections cannot be a playground for President Putin," they wrote.

CNN has reached out to the White House for comment on the letter and has not yet gotten a response.

Trump said Thursday he thinks Russia was behind 2016 election meddling , but added that he feels "it could have been other people in other countries" and that "nobody really knows for sure."

"I think it very well could be Russia but I think it could very well have been other countries," Trump said during a news conference with Polish President Andrzej Duda. "I think a lot of people interfere."

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