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The United Nations top diplomat in Somalia says there already are significant improvements in the security situation in the capital, Mogadishu, after the recent surprise withdrawal of Islamist fighters from the city.

Ambassador Augustine Mahiga told reporters by teleconference from Mogadishu Wednesday that al-Shabab's retreat from the city has fragmented its fighters into three groups, weakening their strength. Mahiga said the Islamist militants also are being starved of financial support from individual benefactors mainly in the Middle East and the Arabian Gulf region. In addition, he said U.N. Security Council sanctions targeting the militants are taking a toll on the fighters' military and economic strength.

However, Mahiga cautioned that warlords are likely to fill the vacuum created by the departure of al-Shabab fighters, unless the Somali government takes immediate action to fill the gap.

Earlier Wednesday, Mahiga briefed the Security Council by teleconference and told members that the improved security situation means the United Nations will be able to expand its presence in Somalia sooner than expected.

Al-Shabab militants withdrew from Mogadishu on Saturday. The group once controlled nearly all of Mogadishu but had steadily lost ground in recent months to government and African Union forces.

Somalia's government has said it is moving to establish security in the capital and will not tolerate what it called attempts by other forces to occupy districts formerly run by al-Shabab.

Al-Shabab continues to control large sections of southern and central Somalia.