President Trump answered questions about Russian interference in the 2016 presidential election in a news conference in Warsaw on July 6. (The Washington Post)

WARSAW — President Trump refused Thursday to say definitively that he believes Russia was responsible for interference in the 2016 U.S. election.

"I think it could very well have been Russia, but I think it could well have been other countries. I won’t be specific," Trump said at a news conference in Warsaw with Polish President Andrzej Duda. "I think a lot of people interfere. I think it’s been happening for a long time."

The U.S. intelligence community has said publicly that it believes Russia was responsible for efforts to meddle in the election and that the operation was directed by Russian President Vladimir Putin.

“Nobody really knows," Trump added. "Nobody really knows for sure.”

Trump's comments come just one day before he is expected to hold a bilateral meeting with Putin during a Group of 20 summit in Germany, and questions remain about whether he will confront Russia over the issue.

In Washington, Senate Minority Leader Charles E. Schumer (D-N.Y.) and a few colleagues plan to release a letter Thursday demanding that Trump press Putin on Russian interference in the election, Schumer's office said.

Trump cited intelligence reports about weapons of mass destruction in the lead-up to the Iraq War as evidence that the intelligence community's findings might not be accurate.

“When I was sitting back listening about Iraq … weapons of mass destruction, how everybody was 100 percent sure,” Trump said. “They were wrong, and it led to a mess.”

But Trump did go on to strongly criticize President Barack Obama, claiming he did “nothing” about the election interference. He claimed that Obama did not act because he believed that the Democratic candidate, Hillary Clinton, would win.

“He did nothing about it,” Trump asserted. “The reason is, he thought Hillary was going to win.”

“Why did he do nothing from August all the way to November 8?” Trump asked. “His people said he choked. I don’t think he choked.”

The Obama administration, however, was deeply engaged in contacts with the CIA and other agencies investigating computer hacking and other measures believed linked to Russia in the months before the election.

A host of punitive actions on Russia were debated by the Obama team. In late December, Obama approved a modest package combining measures that had been drawn up to punish Russia for other issues, including the expulsions of 35 "intelligence operatives" and the closure of two Russian compounds.

The event marked the first time that President Trump has taken questions during an overseas trip.

In his opening remarks, Trump warned that the United States and Europe were united in confronting Russia's "actions and destabilizing behavior."

He also issued a warning to North Korea after the rogue nation successfully tested an intercontinental ballistic missile.

"We will confront it very strongly," Trump said. "There are consequences for their very, very bad behavior."

Later, Trump added that he would not draw "red lines" with North Korea.

"We’ll just take a look at what happens over the coming weeks and months with respect to North Korea," Trump said. "It’s a shame that they are behaving this way."

"But they are behaving in a very, very dangerous manner, and something will have to be done about it," he added.