Somalia ? A major offensive by Somalia’s Islamist rebels is posing the most serious challenge yet to the country’s latest central government, reviving long-standing concerns that the chaotic Horn of Africa nation could fall entirely to al-Qaida-linked militants.

Ten days of heavy fighting across the bombed-out capital of Mogadishu and other areas has pitted the Islamist rebels against the fragile government of President Sharif Ahmed, a widely respected moderate Islamist once vilified by U.S. officials but now regarded by Washington as Somalia’s last, best hope.

In a major blow Sunday, rebels took a key government stronghold, Ahmed’s home town of Jowhar, about 50 miles north of the capital, giving them control of major routes to the north.