Washington: A red-faced State Department said on Friday that the passport files of the three remaining US presidential candidates were all breached as it launched a full probe into the incidents.

In a growing campaign uproar, the State Department found that the files for Democratic Senator Hillary Clinton and Republican Senator John McCain had been breached in a way similar to Democratic Senator Barack Obama's.

US officials said Secretary of State Condoleezza Rice had by early Frirday afternoon telephoned all three candidates to apologise for the incidents which they admitted were embarrassing.

Obama, who was the first to have learned of the breaches, and McCain, who was travelling overseas, both demanded a full investigation of the incidents which overshadowed international diplomacy at the State Department.

Contract staff blamed

"We're going to do a full investigation and that investigation is going to be led by our inspector general here in the Department of State," State Department spokesman Sean McCormack confirmed.

The State Department said on Thursday that it fired two contract employees and disciplined a third over three unauthorised accesses to Obama's file in January, February and March.

"I told him that I was sorry," Rice told reporters after she talked to Obama about the incident. "And I told him that I myself would be very disturbed if I learned somebody looked into my passport file."

The State Department characterises the incidents involving all three candidates as cases of "imprudent curiosity."

The episode raised questions as to whether the actions of the three contractors were politically motivated. Obama and Senator Hillary Rodham Clinton are in a close race for the Democratic presidential nomination.

"None of us wants to have a circumstance where any American's passport files are looked at in an unauthorised way," Rice said.

Rice, who spoke with Obama by phone, said she was particularly disappointed that senior officials at the State Department were not immediately notified.

Meanwhile, Clinton campaign staff said that Rice has told Hillary Clinton her passport file was breached last year.