UNITED NATIONS, May 21 (Reuters) - The top U.N. aid official suggested on Monday that Somalia's government was underestimating the humanitarian crisis there and criticized its plan to bar refugees from living in public buildings.

John Holmes, undersecretary-general for humanitarian affairs, told the Security Council the country's leaders had given far lower estimates than those of the United Nations on how many people had fled the capital during battles in March and April.

Holmes visited the strife-torn Horn of Africa state May 11 and 12, soon after a fresh wave of fighting between Islamist insurgents and the interim government and its Ethiopian allies. His trip was curtailed after bombs planted by suspected insurgents killed at least three people.

Holmes said President Abdullahi Yusuf and Prime Minister Ali Mohamed Gedi had assured him they were committed to helping relief efforts. "However, our discussion was complicated by disagreement on the severity of the crisis," he said.

The United Nations says nearly 400,000 people fled Mogadishu, but Yusuf and Gedi suggested only 30,000 to 40,000 had been displaced and that half of them had already returned, Holmes said.

He said he had also raised the fate of some 250,000 long-term refugees in Mogadishu. Some sites where they were living had now been abandoned while those who had been occupying public buildings could not return because of government plans to repossess the buildings.

"The government has not yet suggested an alternative sustainable solution other than to suggest a return to the areas of origin," said Holmes, a Briton who is the most senior U.N. official to visit the Somali capital in a decade.

The West broadly supports the government but is uneasy at its failure to reach out to the Islamists. There are tensions between the United States and Europe over the degree of support to the government and its Ethiopian backers.

Holmes said public groups he had met in Mogadishu had expressed concerns about intimidation of civil society and the local media and said they feared the United Nations and the outside world had given up on the country.

"We all have a responsibility ... not to turn our backs on Somalis in their latest hour of desperate need," he told the Security Council.

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