The grand jury was expected to convene on Friday morning to consider the first charges in the two-year probe led by special counsel Patrick Fitzgerald.
Legal sources involved in the case said US Vice-President Dick Cheney's chief of staff, Lewis Libby, was among top White House officials likely to be charged with making false statements to the grand jury.
Fitzgerald has also zeroed in on Karl Rove, President George Bush's top political adviser, possibly for perjury. But lawyers involved in the case said Fitzgerald might not be prepared at bring charges against Rove in the initial round.
The New York Times, citing people briefed officially about the case, said Rove would not be indicted along with Libby on Friday but would remain under investigation.
Grand jury's term
Despite their initial denials, it has emerged that both Rove and Libby spoke to reporters in June and July 2003 about the CIA operative, Valerie Plame.
Plame is married to former ambassador Joseph Wilson, who has accused the White House of trying to punish and discredit him over his criticism of the Bush adminstration's use of intelligence to exaggerate the threat posed by Iraq.
It was unclear how Fitzgerald would keep the Rove investigation going since the current grand jury is scheduled to expire on Friday.
Rove may not be indicted, but
will remain under investigation
The New York Times said Fitzgerald was likely to extend its term. But federal guidelines suggest that he would have to seek a new grand jury, because the current one had served the maximum allowable time.
Other current and former administration officials could also face charges on Friday, lawyers said.
Indictments in the case could trigger an immediate shake-up at the White House, already on the defensive over the response to Hurricane Katrina, opposition to the Iraq war and the withdrawal of Bush's nominee for the US Supreme Court, Harriet Miers.
Dark days for Bush
Less than a year after winning re-election by a comfortable margin, President George Bush's approval ratings are at the lowest since he took office in 2001.
On Tuesday the US military death toll in Iraq hit 2000, a fresh reminder of the president's push to war over weapons of mass destruction that were never found.
"There are times when no matter what you do it seems to blow up in your face, whether it's self-inflicted or inflicted from the outside"
Two days later an insurrection of the president's conservative political base forced the withdrawal of Supreme Court nominee Harriet Miers on Thursday.
Adding to Bush's woes is a raft of gloomy economic news, including figures showing consumer confidence dropped, home sales were down and the number of people who lost their jobs because of Hurricane Katrina climbing above the half-million mark.
"There are times when no matter what you do it seems to blow up in your face, whether it's self-inflicted or inflicted from the outside," said Democratic consultant Joe Lockhart, former press secretary to President Bill Clinton during his impeachment trial.