Hillary Clinton, who had been expected to concede the Democratic race to Obama, had earlier said she would not make a decision on whether to continue her campaign.
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"I will make no decisions tonight, " she said in a speech in New York.
Clinton, a New York senator, praised Obama and his supporters "for all they accomplished," saying they had run an extraordinary race for the Democratic presidential nomination.
She said she was committed to uniting the Democratic Party for the November presidential election against John McCain, the presumptive Republican candidate.
'Historic moment'
The US states of South Dakota and Montana voted in the last two Democratic presidential primaries on Tuesday.
If Obama's candidacy is confirmed he will become the first African-American presidential candidate for a major US political party.
Obama won the Montana primary poll after a close race, US media reported.
Hillary Clinton won the South Dakota primary, according to US media predictions. 
Al Jazeera's Rob Reynolds said Obama's projected victory was a historic moment for African-Americans.