The US and the African Union have warned Ethiopia not to withdraw its troops from Somalia before peacekeepers are deployed to replace them.
Some 1,700 AU troops are already in Somalia
AU commission chief Alpha Oumar Konare says it would be a "catastrophe" if Ethiopia pulled out too soon.
US Africa envoy Jendayi Frazer said it would probably be several months before the full peacekeeping force arrived.
Ethiopia's prime minister says he wants to withdraw all his troops, after they helped oust Islamists.
Up to a third of the population fled recent fighting in the capital, Mogadishu, and badly need aid.
Less than 40% of the estimated 300,000 displaced Somalis are receiving any help, according to UN humanitarian relief chief John Holmes.
After a visit to Mogadishu, he said Somalia now represents a worse displacement crisis than Sudan's Darfur region.
Ethiopian Prime Minister Meles Zenawi said he wanted to end the "onerous" financial burden of having Ethiopian troops, estimated to be several thousand in number, in Somalia.
"Things have improved significantly in Mogadishu, making it possible for peacekeeping troops to do their job," he said.
"I very much hope and expect that those African countries that have promised to send troops will do so."
Mr Konare told the AFP news agency that Ethiopia must wait for the AU forces.
"If Ethiopia withdrew from Somalia today, it would be a catastrophe," he said.
He said Ethiopia had done the job of the AU but its continued presence could "block political dialogue" in Somalia.
But Ms Frazer said that an Ethiopian withdrawal before the AU troops arrived would lead to a security vacuum.
"It would be a mistake for Ethiopia to withdraw. That said, we certainly want them to withdraw from Somalia as soon as possible," she told the BBC's Newshour programme.
Ethiopia has twice fought border wars with Somalia and is widely distrusted there.
So far, Uganda has sent 1,700 peacekeepers but the AU says another 6,300 troops are needed.
Nigeria, Burundi and Ghana have all promised to contribute to the AU force.
Mr Meles said the "organised resistance" of the Islamists had now been "broken".
The Ethiopia-backed government says it is in control of Mogadishu after what is seen as the worst fighting in 16 years in the city.
But gunmen attacked a World Health Organization office in the city on Monday night, injuring a guard, WHO officials say.
Aid workers have accused authorities of hindering the passage of food aid at checkpoints set up across Mogadishu.
"We estimate that we are only reach 35 to 40% of those in need," Mr Holmes said.
"Many are already suffering from a cholera outbreak."
Mr Holmes described conditions in Mogadishu as "depressing"
Mr Holmes said international law had been violated by the fighting factions in the city, saying that some citizens had disappeared without explanation.
"Clearly, human rights abuses have taken place, but the government categorically denied reports and accusations of their involvement," he said.
He said the government had promised to co-operate with a planned UN investigation into the reports.
Mr Holmes, the most senior UN official to visit the city in more than a decade, had to cut short his trip on Saturday, after bombs exploded in Mogadishu, killing three people.
Somalia has been without an effective national government for 16 years, controlled by rival militias and awash with guns.