Probable US Democratic presidential nominee Barack Obama has held a surprise meeting with his defeated rival Hillary Clinton.

The unannounced talks between the two political heavyweights came just days after Mr Obama effectively secured the Democratic nomination for the US presidential election in November.

Robert Gibbs, Mr Obama's communications director, confirmed the pair had met on Thursday night to discuss how they could bring their campaigns together and unite the party.

"Senator Clinton and Senator Obama met tonight and had a productive discussion about the important work that needs to be done to succeed in November," he said in a statement.

Mrs Clinton will end her presidential campaign this weekend and officially back Mr Obama, her campaign team announced on Thursday.

In a campaign e-mail sent out to supporters, Mrs Clinton, the senator for New York, said she would host an event in Washington on Saturday to thank her backers and declare her support for Mr Obama.

"On Saturday, I will extend my congratulations to Senator Obama and my support for his candidacy," Clinton said in the e-mail.

"It has been a long and hard-fought campaign, but as I have always said, my differences with Senator Obama are small compared to the differences we have with Senator McCain and the Republicans.

"I have said throughout the campaign that I would strongly support Senator Obama if he were the Democratic Party's nominee, and I intend to deliver on that promise.

"I will be speaking on Saturday about how together we can rally the party behind Senator Obama. The stakes are too high and the task before us too important to do otherwise."

The final primaries of the season took place on Tuesday, with Mr Obama victorious in Montana and Mrs Clinton winning South Dakota.

A candidate needs 2,118 delegates to secure the Democratic nomination. Mr Obama has the support of 2,154 delegates, while Mrs Clinton has 1,919.

Mr Obama, the senator for Illinois, will become the first black US presidential candidate from a leading political party.