The Obama campaign raised the possibility of political motivation

Barack Obama

A spokesman for the department, Sean McCormack, said the cases were probably the result of "imprudent curiosity".

Secretary of State Condoleezza Rice told reporters she had apologised to Senator Obama.

A spokesman for Mr Obama suggested that the government could be using private information for "political purposes".

The BBC's North America editor, Justin Webb, says it is an extraordinary lapse in security which allowed temporary state department employees access to personal information on a man who is guarded by the secret service day and night.

'Security breach'

The state department tracks those who access its passport database.

Breaches occurred on three separate dates - 9 January, 21 February and 14 March.

"We believe this was out of imprudent curiosity, so we are taking steps to reassure ourselves that that is, in fact, the case," Mr McCormack said.

But he said it was not clear what the employees may have seen or what they were looking for.

Ms Rice said she had telephoned Mr Obama to apologise.

"I told him that I was sorry and I told him that I myself would be very disturbed if I learned that somebody had looked into my passport file," she told reporters.

Bill Burton, a spokesman for Democratic contender Mr Obama's campaign, called for a full investigation.

"This is an outrageous breach of security and privacy, even from an administration that has shown little regard for either over the last eight years," he said.

"Our government's duty is to protect the private information of the American people, not use it for political purposes."

"We demand to know who looked at Senator Obama's passport file, for what purpose, and why it took so long for them to reveal this security breach."