President Donald Trump at a news conference with Polish President Andrzej Duda at the Royal Castle on Thursday in Warsaw. AP Photo/Evan Vucci

WARSAW, Poland — On the eve of his first meeting with Russian President Vladimir Putin, President Donald Trump questioned the veracity of American intelligence about foreign meddling in the US election, arguing Thursday that Russia wasn't the only country that may have interfered.

"Nobody really knows for sure," Trump said.

Opening his second overseas trip as president, Trump also warned North Korea that he was considering "some pretty severe things" in response to the isolated nation's unprecedented launch of a missile capable of reaching the US. Though he declined to offer specifics on the US response, he called on all nations to confront the North's "very, very bad behavior."

As US investigations into Russia's meddling move ahead, Trump is under intense scrutiny for how he handles his first face-to-face session with Putin. US intelligence officials say the unpredictable Russia leader ordered interference in the election that brought Trump to the White House.

Trump and Putin plan to sit down on Friday in Hamburg, Germany, on the sidelines of an international summit.

Loathe to cast a shadow on his election victory, Trump has avoided firmly blaming Moscow for campaign hacking in the past, and on Thursday he was similarly elusive. He argued variably that it could have been Russia, probably was Russia, and indeed was Russia while insisting it could have been other countries, too, and adding: "I won't be specific."

"A lot of people interfere," Trump said in Poland. "It's been happening for a long time." He did not specifically say whether he planned to discuss the issue with Putin.

The president sought to redirect any scrutiny toward his predecessor, Barack Obama, accusing him of allowing Moscow to meddle on his watch. Though the Obama administration warned Russia publicly and privately before Election Day to stop interfering, questions have since been raised about whether he acted aggressively enough to stop the threat.

"They say he choked. Well, I don't think he choked," Trump said. "I think he thought Hillary Clinton was going to win the election, and he said, 'Let's not do anything about it.'"

Trump said the CIA had informed Obama about the hacking months before the election but added that "mistakes have been made." Though Trump has made similar statements before, it was an extraordinary public expression of doubt about US intelligence capabilities by a president while standing on foreign soil.

In his first public comments since North Korea tested an intercontinental ballistic missile for the first time this week, Trump declined to offer specifics about what a US response might entail, though he called it a "threat" and said the US would "confront it very strongly." Trump said it wasn't certain he'd follow through on the severe steps he was weighing, adding that he did "not draw red lines."

"It's a shame that they're behaving this way," Trump said of North Korea's leaders. "But they are behaving in a very, very dangerous manner, and something will have to be done about it."

The US has been considering a range of possible sanctions, economic measures, and other steps in response to Pyongyang. The test of an ICBM marked a major technological advancement for North Korea that US officials have described as intensifying the threat against the US by bringing the North closer to being able to mount a nuclear warhead atop a missile that could hit American soil.

Trump's comments came as he opened his second visit to Europe, a trip that will also take him to Germany for the Group of 20 economic summit, where he'll meet with Putin.

Later on Thursday, Trump planned to deliver a speech from Krasinski Square, the site of a monument commemorating the 1944 Warsaw Uprising against Nazi occupation. Polish media reports said the government, as part of its invitation to Trump, promised the White House a reception of cheering crowds. Plans call for ruling party lawmakers and pro-government activists to bus in groups of people from outlying provinces for the speech.

In his speech, Trump planned to call on Poland and all of America's European allies to stand united against extremism and other "shared enemies" that pose a threat to freedom and sovereignty — whether "from the South or the East," according to excerpts of his speech released by the White House in advance.

Trump started his first day in Europe at the Royal Castle, welcomed by President Andrzej Duda and a vigorous handshake in front of a white marble bust of Stanislaw August Poniatowski, the last king of Poland. The leaders then retreated to a room decorated with red walls for their private talks.

___

Associated Press writers Josh Lederman, Vivian Salama and Jill Colvin in Washington contributed to this report.