MOGADISHU -- Four people were injured, two seriously, in an explosion at the residence of Somali Prime Minister Hassan Abshir Farah, police said Saturday.

He was not hurt but four of his staff were hurt in Friday night's blast, which appeared to be caused by either a grenade or a bomb planted in the residence compound, they said.

"We are investigating the matter. Preliminary reports indicate that it was an attack," said Mogadishu police chief Abdi Hassan Awale Qeybdid, who was at the compound at the time of the blast.

Security was stepped up at the residences of all members of the Transitional National Government (TNG) following the explosion.

The TNG, set up in 2000, is opposed by most of the country's warlords, who have perpetuated a state of anarchy and violence in the horn of Africa country since dictator Mohamed Siad Barre was toppled in 1991, AFP reported.