Washington - In the first extensive interview since his wife lost the bruising presidential primary elections, former US president Bill Clinton called Barack Obama "smart" but warned Americans that voting across the racial divide was not enough, the Washington Post reported Sunday.

Clinton gave the 45-minute interview in Kigali, Rwanda, where he was visiting some of the health and agriculture projects supported by his foundation.

Clinton declined to discuss criticism of some of his actions and comments - especially about Barack's racial advantage among black voters in the south - during Hillary Clinton's bid to become the country's first female presidential candidate, which some have said undermined her campaign, the Post reported.

Obama, set to become the country's first African American presidential nominee when the Democratic Party holds its convention later this month, has based much of his pitch to voters on the idea of "one America" that bridges all ethnic, social and religious groups.

Hillary Clinton charged during the close campaign that Obama's idealistic themes were empty rhetoric.

The former president again lanced the racial issue in the Post interview, warning Democrats that they can't build "one America on the cheap."

He said people could not tell themselves, "'I voted across the racial divide; I have no obligations to do something in my community or around the world.' In other words, if (Obama) wins . . . we've still got a lot of problems. We've got to heave-to here. We've got to show up.'"

Hinting at unsettled issues from the primary campaign, Clinton told the Post reporter: "Next year, you and I and everybody else will be freer and have more space to say what we believe to be the truth" about the primaries.

Democrats are trying to show a united front in order to regain the White House after eight years of President George W Bush, a Republican. (dpa)