Special counsel Robert Mueller said Wednesday he believed he was constitutionally barred from charging President Donald Trump with a crime but pointedly emphasized that his Russia report did not exonerate the president.

"If we had had confidence that the president clearly did not commit a crime, we would have said so," Mr. Mueller said. "We did not however make a determination as to whether the president did commit a crime."

He cautioned lawmakers who have been negotiating for his public testimony that he would not go beyond his report in the event he appears before Congress. But he also signaled that Congress was the proper venue, not the criminal justice system, for deciding whether action should be taken against the president in connection with allegations that Mr. Trump and aides obstructed the investigation of Russian interference to help the Republicans in the 2016 election campaign.

Mr. Trump, who has repeatedly and falsely claimed that Mr. Mueller's report cleared him of obstruction of justice, modified that contention somewhat shortly after the special counsel's remarks. He tweeted, "There was insufficient evidence and therefore, in our Country, a person is innocent. The case is closed!"

Mr. Mueller's comments were his first public statements since his appointment as special counsel two years ago. His remarks, one month after the public release of his report on Russian efforts to help Mr. Trump win the presidency, appeared intended to both justify the legitimacy of his investigation against complaints by the president and to explain his decision to not reach a conclusion on whether Mr. Trump had obstructed justice.

Indicting Mr. Trump, he said firmly, was "not an option" in light of a Justice Department legal opinion that says a sitting president cannot be charged. But, he said, the absence of a conclusion should not be mistaken for an exoneration of the president.

"The opinion says the Constitution requires a process other than the criminal justice system to formally accuse a sitting president of wrongdoing," Mr. Mueller said, referring to the Justice Department legal opinion. That would shift the next move, if any, to Congress, and the chairman of the House Judiciary Committee, which would investigate further or begin any impeachment effort, commented quickly.

It falls to Congress to respond to the "crimes, lies and other wrongdoing of President Trump - and we will do so," said New York Rep. Jerrold Nadler.

Mr. Trump has blocked the committee's subpoenas and other efforts to dig into the Trump-Russia issue, insisting Mr. Mueller's report has settled everything.

Mr. Mueller's statement came amid demands for him to testify on Capitol Hill about his findings and tension with Attorney General William Barr over the handling of Mr. Mueller's report.

That report found no criminal conspiracy between the Trump campaign and Russia to tip the outcome of the 2016 presidential election in Mr. Trump's favor over Democrat Hillary Clinton. But it also did not reach a conclusion on whether the president had obstructed justice.

Mr. Barr has said he was surprised Mr. Mueller did not reach a conclusion on whether the president had criminally obstructed justice, though Mr. Mueller in his report and again in his public statement Wednesday said that he had no choice. Mr. Barr decided on his own that the evidence was not sufficient to support an obstruction charge against Mr. Trump.

"Under longstanding department policy, a president cannot be charged with a federal crime while he is in office," Mr. Mueller said. "That is unconstitutional. Even if the charge is kept under seal and hidden from public view that, too, is prohibited."

Mr. Mueller, for his part, complained privately to Mr. Barr that he believed a four-page letter from the attorney general summarizing his main conclusions did not adequately represent his findings.

Mr. Mueller also appeared to put Congress on notice that he would not break new ground in the event he testifies on Capitol Hill.

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"I do not believe it is appropriate for me to speak further about the investigation or to comment on the actions of the Justice Department before Congress."

This story was reported by The Associated Press. Associated Press writers Chad Day, Mary Clare Jalonick and Lisa Mascaro contributed to this report.