The endorsement will be announced on Monday in Washington, said the officials, speaking on condition of anonymity because they were not authorised to speak for the record. An official close to the senator said the announcement will be made during an Obama campaign rally at American University, where he will be joined by Sen Kennedy and his niece, Ms Caroline Kennedy, who also has endorsed Mr Obama.

In a television interview Sunday, Mr Obama would not answer questions about an endorsement from Mr Kennedy.

'I'll let Ted Kennedy speak for himself. And nobody does it better,' he said on ABC's 'This Week.' 'But obviously, any of the Democratic candidates would love to have Ted Kennedy's support. And we have certainly actively sought it. And you know, I will let him make his announcement and his decision when he decides it's appropriate.'

Mr Kennedy's endorsement was highly sought after by all the Democratic candidates. Besides his status as a liberal icon and member of the Kennedy dynasty, Mr Kennedy boasts a broad national fundraising and political network as well.

Mr Kennedy is friendly with the Democratic candidates. Mr Obama and Mrs Hillary Rodham Clinton both serve on the Senate Health, Education, Labour and Pensions Committee, and Mr Kennedy is chairman.

Former Sen John Edwards partnered with Mr Kennedy on patients' rights legislation in 2001, and Mr Kennedy was a key congressional ally when President Bill Clinton was in office.

Difficult to choose
'It's going to be difficult choosing,' Mr Kennedy said in October.

'I've got a lot of friends who want to be president.' Mr Kennedy's endorsement of Mr Obama will follow that of his niece, Caroline, who backed the Illinois senator in an opinion piece published Sunday in the New York Times. She said Mr Obama could inspire Americans in the same way her father, President John F. Kennedy, did.

'I have never had a president who inspired me the way people tell me that my father inspired them,' she wrote. 'But for the first time, I believe I have found the man who could be that president - not just for me, but for a new generation of Americans.'

Ms Caroline Kennedy, who was four days shy of her 6th birthday when her father was assassinated in 1963, wrote that Mr Obama 'has a special ability to get us to believe in ourselves, to tie that belief to our highest ideals and imagine that together we can do great things.' And she appealed to other parents to pick a candidate who she said could invigorate a younger generation that is too often 'hopeless, defeated and disengaged.'

Mr Kennedy wrote that she wants a president 'who appeals to the hopes of those who still believe in the American Dream, and those around the world who still believe in the American ideal; and who can lift our spirits, and make us believe again that our country needs every one of us to get involved.' -- AP