President George W Bush says he remains focused in his work despite the charges against a top White House official.
Lewis Libby is a very influential figure
Lewis Libby, the vice-president's chief of staff, resigned after being charged with perjury over the inquiry into the unmasking of a covert CIA agent.
Mr Bush said he was saddened by the news, but would continue to work hard to protect the American people.
The identity of CIA agent Valerie Plame - whose husband criticised the Iraq war - was leaked to a US reporter in 2003.
Special prosecutor Patrick Fitzgerald has accused Mr Libby of lying to investigators about how and when he learned and disclosed to reporters classified information about Ms Plame.
LEWIS LIBBY INDICTMENT
Two charges of perjury
Two counts of making a false statement
One charge of obstruction of justice
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If found guilty on all five counts in the indictment, Mr Libby, 55, faces a maximum of 30 years in prison and a $1.25m (£705,000) fine for each charge.
He said he was confident he would be "completely and totally exonerated".
Mr Bush's top aide Karl Rove was not charged but the investigation remains open.
Opposition Democrats think they have the administration on the run, says the BBC's Washington correspondent Justin Webb.
Defeated presidential candidate John Kerry has complained of corruption at the highest levels of government, far from the honour and dignity America was promised.
Speaking outside the White House, President Bush said Mr Libby - also known as Scooter - had "worked tirelessly on behalf of the American people and sacrificed much in the service to this country".
"He served the vice-president and me through extraordinary times in our nation's history."
Karl Rove may have to testify against his former colleague
Mr Bush said Mr Libby could expect "due process and a fair trial".
Mr Fitzgerald's investigation and ongoing legal proceedings were "serious", Mr Bush said, but his team remained "wholly focused on the many issues and opportunities facing this country.
"I got a job to do, and so do the people who work in the White House. We got a job to protect the American people, and that's what we'll continue working hard to do."
The indictment is not a knock-out blow, says our Washington correspondent.
But the president is unquestionably politically wounded and he will suffer further political embarrassment in the court case which will follow, he says.
The indictment refers to a person known as "Official A", who is said to be a senior White House player and who appears to be a likely courtroom witness against Lewis Libby.
Most people believe it is Karl Rove, and that of course would present horrible difficulties for the White House, says our correspondent.
Vice-President Dick Cheney may also have to testify at a trial. Lewis Libby is much more important than his bland title of chief of staff to the vice-president suggests, says our correspondent.
Mr Libby has been a prominent backroom hawk in the Bush team, an enthusiast for the Iraq war and a man very heavily involved in efforts before the war to make the case to the public that Saddam Hussein had to go, he says.
This latest crisis for the Bush administration follows the withdrawal on Thursday of the president's nominee for the Supreme Court, Harriet Miers, after criticism from Mr Bush's conservative supporters.
Ms Plame's identity was leaked after her husband, diplomat Joseph Wilson, accused the Bush administration of manipulating intelligence to support military action against Iraq.
Mr Wilson says it was done to undermine his credibility. Others have raised the possibility that it was a form of payback for her husband's criticism.
Mr Wilson has told American television that since her identity was revealed there have been specific threats against her.