(Fixes typo in penultimate paragraph)
* Obama administration has appointed two prosecutors
* Lieberman says this does not go far enough
* White House defends "very thorough investigation"
WASHINGTON, June 17 (Reuters) - A Senate committee chairman called on Sunday for a special counsel to investigate suspected leaks of U.S. classified information following allegations that the White House made the disclosures to boost President Barack Obama's election chances.
Senator Joseph Lieberman, chairman of the Senate Homeland Security and Governmental Affairs Committee, said U.S. Attorney General Eric Holder's appointment on June 8 of two federal prosecutors to investigate the suspected leaks did not go far enough.
"We need a special counsel because a special counsel avoids any appearance of conflict of interest," Lieberman said on "Fox News Sunday."
"Special counsels - independent counsels before them - were created for situations exactly like this where people might reach a conclusion that investigators, U.S. attorneys even, working for the attorney general - who was appointed by the president - cannot independently and without bias investigate high officials of their own government."
The secrets, revealed in media stories, have included reports on U.S. cyber warfare against Iran, procedures for targeting militants with drones and the existence of a double agent who penetrated a militant group in Yemen.
Republicans have already demanded an outside special counsel to investigate the leaks. Lieberman is an independent who caucuses with the Democrats in the Senate and usually votes with them.
Holder said the investigation would be headed by U.S. Attorneys Ronald Machen Jr. of Washington and Rod Rosenstein of Maryland, who would be "fully authorized to prosecute criminal violations discovered as a result of their investigation."
Senior White House adviser David Plouffe defended the Obama administration's approach in the leaks investigation.
"This ought to be investigated as thoroughly as anything can and we ought to wait for the results of that investigation," Plouffe said on the same program.
"There's going to be a very thorough investigation," he added.
Plouffe was asked if Obama would sit down with the investigators but declined to answer. Asked if Obama had declassified the information that ended up in published reports, Plouffe said, "No, of course he didn't."
The investigation by the two federal prosecutors is likely to include scrutiny of White House officials, sources have told Reuters.
The issue has spilled into the presidential campaign, with some Republicans charging the leaks appear calculated to boost the Democratic president's re-election prospects on Nov. 6. Obama has said he has "zero tolerance" for these kinds of leaks.
Lieberman said the leaks were "the worst in a long time" and called for stronger U.S. laws to prevent future leaks.
"In my opinion, an enormous amount of damage has been done to our national security," Lieberman said. "In the case of the cyber attack on Iran ... this is the first confirmation of that. Some methods of how it was carried it out were telegraphed to the Iranians. I think there's a danger that it may legitimize an Iranian or terrorist counter-cyber attack on us."
Lieberman said the leaks had also angered the operative who infiltrated Yemen-based al Qaeda in the Arabian Peninsula.
"This will discourage people and foreign intelligence services from cooperating with us in the future," Lieberman said. (Reporting by Will Dunham, Bill Trott, David Brunnstrom and Thomas Ferraro; Editing by David Brunnstrom)