Barigye Ba-Hoku told Voice of America a mortar round fired by al-Shabaab militants exploded near a contingent of Ugandan peacekeepers guarding the palace compound in the Somali capital.
Ba-Hoku called the strike Monday a "lucky hit" for the al-Qaida-linked militants who want to overthrow Somalia's U.N.-backed government and force the withdrawal of peacekeepers.
Ba-Hoku dismissed claims by al-Shabaab that its fighters are closing in on a major road that links the presidential palace to Mogadishu's airport, CNN reported.
Last week al-Shabaab vowed to wage a massive final war against the Somali government and the African Union troops.
On Friday an al-Shabaab spokesman said militants had captured government military bases and the Mogadishu base of the pro-government Sufi Muslim militia.