Somali Islamist militants al-Shabaab have claimed responsibility for the two bomb attacks in the Ugandan capital Kampala that killed more than 70 people.

The attacks took place during the final moments of Sunday's World Cup and are the first time that the al-Qaida-linked al-Shabaab has hit outside Somalia.

At least 28 Ugandans and 11 Ethiopians are among the dead, as well as an Irish woman and an American charity worker, according to the Ugandan government.

Thirty-three of the dead have not yet been identified.

al-Shabaab spokesman Sheik Ali Mohamud Rage claimed responsibility two days after the group threatened revenge attacks on Uganda and Burundi for their contribution to the African Union peace keeping force that prop up the Somali government.

US President Barack Obama has condemed the attacks as deplorable and cowardly and pledged support to the Ugandan government to find those responsible.

The explosions ripped through two bars packed with soccer fans watching the final moments of the World Cup final on television in an Ethiopian-themed restaurant and at a gathering in a Kampala rugby club on Sunday.

Ugandan officials suspect that both attacks were suicide bombs.