President Donald Trump says the decision to appoint a special counsel to oversee the inquiry into Russian influence on his election “hurts our country terribly”.
He said the US was being made to look “divided, mixed up”, media reported.
Earlier, he tweeted that the decision was “the single greatest witch hunt of a politician in American history”.
Former FBI director Robert Mueller has been selected to lead the inquiry.
“I believe it hurts our country terribly, because it shows we’re a divided, mixed-up, not-unified country,” Mr Trump told CNN and CNBC .
His reaction differed from Wednesday when he was quoted in a White House statement as saying: “I look forward to this matter concluding quickly.”
In naming Robert Mueller, the US deputy attorney general said it was in the public interest to pick an outsider.
Mr Mueller’s appointment has been welcomed by politicians from both sides.
Calls for a special investigation mounted after Mr Trump fired FBI Director James Comey last week.
“With all of the illegal acts that took place in the Clinton campaign and the Obama Administration, there was never a special counsel appointed,” he tweeted on Thursday.
A number of times previously, Mr Trump has likened the investigation into potential collusion between his campaign and the Russian government to a witch hunt.
His latest tweets came hours before he was to hold a joint press conference with the visiting Colombian president.
On Wednesday, Mr Trump said no politician in history “has been treated worse or more unfairly” than himself.
The announcement of a special counsel apparently took the White House by surprise, with Mr Trump only being informed of it after Deputy Attorney General Rod Rosenstein had signed the order.
The FBI and Congress are looking into potential links between Mr Trump’s campaign team and Russia. Mr Mueller will take over the FBI investigation.
US intelligence agencies believe Moscow tried to tip November’s presidential election in favour of Mr Trump.
Meanwhile, the New York Times reports that former Trump aide Michael Flynn told his transition team at the beginning of January – earlier than was previously thought – that he was under federal investigation for working as a paid lobbyist for Turkey during the election campaign.
Mr Trump appointed Mr Flynn as his national security adviser weeks later despite the warning, but he was sacked after just 24 days.
In his statement announcing Mr Mueller’s appointment, Mr Rosenstein said: “The public interest requires me to place this investigation under the authority of a person who exercises a degree of independence from the normal chain of command.”
Mr Mueller, who will have wide-ranging powers, said simply: “I accept this responsibility and will discharge it to the best of my ability.”
Just over an hour after the news of Mr Mueller’s appointment emerged, President Trump predicted the new investigation would clear him and his team.
“A thorough investigation will confirm what we already know – there was no collusion between my campaign and any foreign entity,” said the president.
The stakes just rose: analysis by Anthony Zurcher, BBC News, Washington
In announcing the appointment of a special counsel, Mr Rosenstein cited the “unusual circumstances” of the ongoing FBI Russia investigation. That’s an understatement.
The circumstances are not just unusual, they are unprecedented. The nation has never had an administration so embattled so early in its term. There have never been such grave allegations of electoral meddling by a foreign power in a US presidential election.
Then again there has never been a president quite like Donald Trump.
Now the Russia story enters a new, more serious phase. Robert Mueller has a sterling reputation in Washington, DC. He worked with Mr Comey when the latter served as deputy attorney general in George W Bush’s administration. He understands pressure-cooker politics and knows how to navigate the corridors of power.
He has wide latitude to conduct his investigation and bring criminal charges, if necessary.
While Mr Mueller is technically still part of the justice department and ultimately reports to Mr Trump, his stature is such that he is unlikely to be cowed by the president.
Independent investigations often take on a life of their own and can reach unexpected conclusions. With Mr Mueller in the game, the stakes just went up.