He also stressed the UN "would not be deterred from delivering its mandate" in Somalia, Mr Ban's spokesman said.

The attackers detonated a car bomb outside the UN mission in the capital Mogadishu, then engaged security forces in a fierce gun-battle.

The al-Qaeda-linked group al-Shabab later said it was behind the attack.

This is the first time the UN offices have been attacked since it recently relaunched its mission in Somalia.

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Analysis

Mark DoyleBBC International Development Correspondent

The UN compound in Mogadishu is just a few hundred metres from the airport where thousands of African Union troops are based. It is in the heart of one of the most sensitive and theoretically well-guarded areas of the city. But following the audacious attack the gateway into the compound is now a wreck of twisted metal and rubble.

For more than a year now the Somali government - and its Western and African backers - have been lauding improved security in Somalia. At a diplomatic conference in London in February, hosted by the British Prime Minister David Cameron, there was optimistic talk about the new government which was for the first time elected by a parliament.

It is true that the militant Islamist al-Shabab group has been forced, by African Union troops, to leave most of their military positions in the city. But this attack shows that opponents of the government still have the capacity to hit a high-profile target which should be one of the best protected locations in the country.

Mr Ban was "deeply concerned and outraged by the despicable attack" on the UN Development Programme office, his spokesman Martin Nesirky said.

The raid was also condemned by the 15-member UN Security Council, which reiterated its "determination to combat all forms of terrorism".

UN envoy to Somalia, Nicholas Kay, who was not hurt in the attack, told the BBC the world body would continue its mission in helping Somalia build peace and development.

He said Somalia had "turned a corner" after decades of conflict, and al-Shabab attacks like the one of Wednesday were "acts of desperation".

"At this stage our firm intent is to stick to the mission and not abandon Somalia," he said.

He added that one UNDP international staff member, three contractors working for a South African company and four Somali guards were killed in the attack.

All seven al-Shabab gunmen died in the raid and officials said pro-government forces later secured the compound.

The UN has only recently expanded its operations in Mogadishu after years when its Somalia mission was based in neighbouring Kenya because of security fears.

The UNDP office is next to the heavily fortified airport in southern Mogadishu.

The security forces arrived at the scene soon after the explosion. The UNDP compound is next to the airport where the AU forces have a base.

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Somali Prime Minister Abdi Farah Shirdon Saaid praised the speed of the security forces' response.

"Somali and Amisom security forces responded immediately to the situation after the initial explosion and have brought the situation under control. Sadly we must wait to hear the full details and confirmation of any casualties," he said in a statement.

"All our thoughts and prayers are with our UN colleagues today. But al-Shabab will not derail the peace process. They will not stop our recovery. Violence will not win."

Al-Shabab, which had been in control of parts of Mogadishu for more than two years, withdrew in August 2011 under pressure from pro-government forces, but continues to launch occasional suicide attacks in the city.

It has also been pushed out of other cities, but still remains in control of smaller towns and large swathes of the countryside in central and southern Somalia.

The improving security situation has prompted the return of Somalis from the diaspora and allowed UN agencies and foreign embassies to return to the country.

Some 18,000 AU troops are in the country supporting the government of President Hassan Sheikh Mohamud who was elected by MPs last September.

His administration is the first one in more than two decades to be recognised by the US and the International Monetary Fund (IMF).