Drought in Somalia is threatening two million people, the vast majority living in zones controlled by radical Islamist insurgents where it is impossible to send aid, a UN official said Thursday.

"Somalia hosts one of the largest populations of displaced people in the world," Valerie Amos, Under-Secretary General for humanitarian affairs and emergency relief coordinator told journalists here.

The country "is again teetering on the brink of a much larger scale disaster due to the threat of a countrywide drought," she said. "Two million people, about 27% of the population, are in crisis."

Amos the full extent of the drought would depend on the main rains due in April.

Difficulty in accessing areas of south and central Somalia controlled by the insurgent Shebab makes it difficult to evalute the exact needs of the people living there, she said.

"The lack of security makes it very difficult for us to carry on our humanitarian work. We do not have access to all the populations we need to have access to, to enable us to meet their needs."

The UN official said she feared a crisis within the country's transitional government could worsen the humanitarian situation in the Horn of Africa country.

"My big concern is that instability in the political process will lead to enhanced conflict and therefore will lead to enhanced humanitarian needs," she said.

Somalia has lacked an effective central government since the ouster of president Mohamed Siad Barre 20 years ago unleashed bloody violence.