NAIROBI, Kenya — Somali officials acknowledged on Thursday that members of Somalia’s presidential guard had defected to the Shabab, the radical Islamist insurgent group that claimed responsibility for the recent bombings in Uganda that killed more than 70 people watching the final game of the World Cup.

The defection of some of the president’s best-trained men is the latest setback for Somalia’s beleaguered transitional government, which has lost important pieces of territory in the past few days. Insurgents are now 300 yards — a rifle shot away — from the presidential palace.

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