The agency, which is providing relief to hundreds of thousands people affected by flooding, said land operations had not been affected.
Floods have killed at least 141 people in Somalia since October when heavy rains sparked flooding across the south and centre of the country.
Somali government forces backed by Ethiopian troops, tanks and aircraft were poised to take control of the capital Mogadishu on Friday.
"The Islamic Courts used fear and force to take over Mogadishu and other areas"
Mack Rogers, Chattanooga, TN, US
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The city was said to be calm but tense after a day of gunfire and looting.
As Ali Mohamed Gedi, the Somali prime minister, prepared to enter Mogadishu, Ethiopian tanks and hundreds of soldiers were camped in the heart of the city amid fears of renewed street anarchy.
Abdirahman Dinari, a government spokesman, said on Friday: "Our forces have entered the city, today we will move on and try to control the whole of Mogadishu. The people of Mogadishu have welcomed us and we will not hesitate to help restore law and order there."
Many Islamic Courts fighters have fled
Dinari said the city's elders had assured the government of their support, but uncertainty still clouded the capital, known for its spontaneous bursts of violence.
Many Islamic courts fighters have swapped their uniforms for civilian clothes and are visible on the streets.
Yusuf Abdulkadir, a resident, said: "The night here in Mogadishu was quiet, but we do not know about today. Mogadishu is clearly an unpredictable city. It can be quiet now, but you can't tell what will happen next."