U.N. Secretary-General Ban Ki-moon is calling for a surge of thousands of African troops in Somalia to stem the threat of terrorism posed by Islamist insurgents and ensure the survival of a U.N.-backed government, whose success Washington believes is crucial to defeating extremist groups in the region, according to a previously undisclosed appeal.

In a letter to the U.N. Security Council, Ban urged member nations to provide the African Union Mission in Somalia financial and military support. He warned that there is an urgent need to strengthen the military campaign against al-Shabab, a Somali militant group linked to al-Qaida, and to "avoid further reversals."

The African Union force is made up of 18,000 troops and is dominated by Ugandans and Burundians. Ban is calling for as many as 4,400 additional troops and support staff for a period of up to two years, and for a limited package of nonlethal support for 10,000 front-line Somali troops.