Somalia's al-Shabab insurgents suffered a major defeat Wednesday when they were forced to surrender control of Baidoa, the country's third largest city that was considered an important rebel base.

The fall of Baidoa in the face of an offensive by forces belonging to the African Union mission in Somalia came on the same day the U.N. Security Council voted to expand that mission by nearly 6,000 troops.

America's U.N. ambassador, Susan Rice, said the council's unanimous vote could, in her words, “make a decisive difference in weakening terrorism and bringing peace.”

A spokesman for Somalia's Transitional Federal government said the al-Qaida-linked militants withdrew from Baidoa without a fight as truckloads of Ethiopian and Somali troops approached. But a rebel spokesman said it was a tactical retreat and the battle to turn Somalia into a strict Islamic state will continue.

Representatives of more than 40 governments and international organizations are gathering in London for a one-day conference Thursday aimed at establishing a stable government in Somalia. The Horn of Africa country has not seen peace for more than two decades.

Among those attending the meeting are U.N. Secretary-General Ban Ki-moon and U.S. Secretary of State Hillary Clinton.

The fall of Baidoa, a city long considered an al-Shabab stronghold, continues the steady loss of its control of much of southern and central Somalia. AMISOM troops recently pushed the rebels out of Mogadishu, the capital.