Washington - US President Barack Obama warned Africans on Tuesday that groups like Al-Qaeda saw their "innocent" lives as cheap, in a personal challenge to extremists on the continent after the Uganda bombings.

A US official meanwhile branded Al-Qaeda, linked to the Somalia-based Shebab group which claimed the attacks, as "racist", as the United States cranked up its diplomatic response to increasingly active extremists in Africa.

Obama, leveraging his African heritage and popularity on the continent, took direct aim at the Shebab and Al-Qaeda after attacks on crowds in Kampala glued to the World Cup final on Sunday killed at least 76 people.

"What you've seen in some of the statements that have been made by these terrorist organisations is that they do not regard African life as valuable in and of itself," Obama told the SABC.

"They see it as a potential place where you can carry out ideological battles that kill innocents without regard to long-term consequences for their short-term tactical gains," he said, in the interview to be broadcast early on Wednesday.

Obama's intervention marked the first, direct comments by the president, whose father was Kenyan, on the Kampala bombings.

A senior American official made clear Obama was taking a direct swipe at the ideology and motives of Al-Qaeda affiliates on the continent, which US intelligence agencies say are the extremist group's most active franchises.

"The president references the fact that both US intelligence and past Al-Qaeda actions make clear that Al-Qaeda and the groups like (Shebab) that they inspire - do not value African life.

"In short, Al-Qaeda is a racist organisation that treats black Africans like cannon fodder and does not value human life," the official said, on condition of anonymity.

US officials drew parallels between the Uganda attacks and the 1998 bombings of American embassies in Kenya and Tanzania, which killed hundreds of Africans, to suggest Al-Qaeda viewed people on the continent as acceptable casualties of its wider goals.

A senior US official also said that US intelligence analysts had concluded that Al-Qaeda leadership figures had specifically targeted black Africans to become suicide bombers.

They did so, the official said in the belief that social and economic conditions on the continent made them more susceptible to recruitment.

Earlier, a separate administration official validated the Shebab's claims to have carried out the bombing, and expressed fears the group could seek to carry out attacks outside of Africa.

The official, also speaking on condition of anonymity, said that though the United States had tracked the rebel group and knew about its Al-Qaeda links, it had no forewarning of the strikes in Kampala.