The head of the UN's refugee agency on Friday called for humanitarian workers to be given access to all parts of Somalia, where conflict and drought have left millions facing starvation.

"The situation that draws the most attention (among) member states is Somalia," said UN High Commissioner for Refugees Antonio Guterres after a meeting of the agency's executive committee.

He described the drought in Somalia as the world's worst ongoing humanitarian catastrophe and expressed his "deep gratitude" to those countries that have kept their borders open to allow the influx of refugees.

Somalia is the worst hit of several East African countries affected by the regions' worst drought in decades.

"There is no humanitarian solution to this problem, it is political," Guterres said.

The UNHCR is assisting some 800,000 Somali refugees in neighbouring countries but it is unable to provide relief to an estimated 3.7 million people in need of urgent help inside Somalia.

The capital Mogadishu is officially under the control of forces supporting the Western-backed transitional government but security has yet to be restored to a level that allows a large-scale humanitarian response.

A suicide bomber from the Al-Qaeda-affiliated Shebab group blew up a truck at a government compound on Tuesday, killing at least 82 people in Somalia's deadliest such attack since the country plunged into chaos two decades ago.

Many other regions affected by drought are still under the control of the Shebab and an effective aid effort there has remained impossible since famine started spreading in July.

The UNHCR's executive committee this week adopted a budget of 3.59 billion dollars for the agency in 2012.