A Washington newspaper reports that a U.S. drone aircraft fired on two leaders of the al-Qaida-linked militant group al-Shabab in Somalia last week, apparently wounding them.

The Washington Post quotes an unnamed senior military official familiar with the operation.

The official said the strike was carried out amid rising concerns that al-Shabab is working more closely with al-Qaida to strike outside Somalia's borders.

Al-Shabab has been primarily active within Somalia, where it is fighting to overturn the transitional government and impose strict Islamic law. However, the group claimed responsibility for bombings in Uganda last July that killed more than 70 people.

The reports says Somalia is the sixth country where the United States is using drones to conduct attacks. The Washington Post says the others are Afghanistan, Pakistan, Libya, Iraq and Yemen.

Earlier this week, U.S. officials said the Pentagon is sending four drones to the region to help battle militants in Somalia.

Military officials say the defense department is sending the drones to Uganda and Burundi, who have supplied troops to the African Union peacekeeping force helping the Somali government.

Al-Shabab rules much of southern and central Somalia and part of the capital Mogadishu.

In February, the government backed by African Union forces launched a new offensive against the group, and since then have retaken parts of the capital.

Somalia has not had a functioning central government since 1991.