Mr. Trump’s decision to weigh in was earlier reported by Fox News.
The move prompted criticism from congressional Democrats, including Representative Adam B. Schiff of California, the ranking member of the Intelligence Committee, who said Mr. Trump’s push to release the informant from the confidentiality agreement should be a “key part” of the new investigation.
It also drew fire from some legal experts who said that it broke with longstanding norms meant to insulate the Justice Department from presidential interference in criminal matters, particularly those with a political dimension.
“The White House is never supposed to meddle in this kind of thing,” said Matthew Miller, a Justice Department spokesman under President Barack Obama. “That is especially so when there is a political consideration, like if you want somebody freed from his confidentiality agreement because you want him to go say ugly things about a political opponent.”
Kellyanne Conway, the counselor to the president, told CNN on Friday that Mr. Trump “believes, as many others do, frankly, that the F.B.I. informant should be free to say what he knows.” She added, “It is not unusual for a president to weigh in.”
Republicans have long tried to link Hillary Clinton, Mr. Trump’s rival as the 2016 Democratic presidential nominee, to the uranium deal, which occurred while she was secretary of state. The agreement required the approval of the Committee on Foreign Investment in the United States, on which the State Department has a voting seat. And the Clinton Foundation received large donations from the chairman of Uranium One and others connected to the company as the deal was unfolding.
Republicans have made unproven allegations that Mrs. Clinton was part of a quid pro quo in which the Clinton Foundation received donations in exchange for supporting the deal. Mrs. Clinton’s presidential campaign and State Department officials have denied that she was involved in the decision, and said that all other members of the committee had signed on regardless.
In recent weeks, the issue has been revived by The Hill newspaper. It reported that before the deal was completed, the F.B.I. had gathered evidence that Russian officials were engaged in bribery, extortion and money laundering to expand Moscow’s nuclear interests in the United States, and had specifically sought access to Mrs. Clinton and her husband, former President Bill Clinton, through contributions to the foundation that bears their name.
“I think the uranium sale to Russia and the way it was done — so underhanded, with tremendous amounts of money being passed — I actually think that’s Watergate, modern age,” Mr. Trump told reporters on Wednesday.
At the same time, the president has worked to shift the focus from his campaign’s dealings with Russia to those of Mrs. Clinton’s, after it was revealed that her presidential campaign and the Democratic National Committee paid for research that was included in a dossier made public in January that contained salacious claims about connections among Mr. Trump, his associates and Russia.
“It is now commonly agreed, after many months of COSTLY looking, that there was NO collusion between Russia and Trump,” he said in a Twitter post on Friday. “Was collusion with HC!”
Ms. Sanders echoed that assertion and said that the White House believed there was “a lot of cause for concern” about the uranium deal.
“We’re starting to now see that all of the things that the Democrats had accused this president of doing, they were actually guilty of themselves,” she said.