* Four of the dead were UNICEF staff

* Top U.N. envoy for Somalia brands attack "despicable"

* Al Shabaab militants have stepped up attacks this year (Updates death toll, U.N. officials, president's comments)

By Abdiqani Hassan

BOSASSO, April 20 (Reuters) - Somali militant group al Shabaab bombed a minivan carrying staff to a United Nations office in the semi-autonomous Puntland region on Monday, killing six people including four from the global body's children's fund UNICEF, officials said.

Images posted on social media show a blood-spattered white vehicle, its windows shattered and the roof blown off by the blast in the region's administrative capital Garowe.

UNICEF said in a statement the improvised explosive device attack occurred when its staff were travelling to the office from their guest house nearby and that four more of its staff were seriously wounded.

"Our immediate thoughts are with the families of the four staff members who were killed and with those who were injured. All of us at UNICEF are deeply saddened and deeply angered," said Anthony Lake, its executive director.

Nicholas Kay, the U.N. Special Representative for Somalia, said two guards also died in the attack.

"The complete disregard for the lives of people working on the humanitarian and development needs of the people of Somalia is despicable," he said in a statement.

Al Qaeda-linked al Shabaab has staged a string of attacks in Somalia and surrounding countries aimed at imposing its harsh brand of Islam and overthrowing the Somali government, which is backed by Western donors and African peacekeepers.

"We are behind the Garowe attack," al Shabaab's military operations spokesman, Sheikh Abdiasis Abu Musab, told Reuters.

Mohamed Abdi, a police officer at the scene of the attack, earlier put the number of dead at nine, comprising Kenyans and Somalis, but UNICEF said its staff hailed from a range of countries. The attacker also died, police said.

Al Shabaab, which once ruled much of Somalia, has been driven out of major strongholds in military offensives launched last year by the Somali army and African Union peacekeepers.

Puntland, on the northeastern tip of Somalia, had in the past largely avoided the regular attacks seen in other areas. However, attacks by the militant group have intensified this year.

"Al Shabaab is not interested in the future of Somalia; it is only interested in pushing its own extremist agenda for its own perverted purposes," President Hassan Sheikh Mohamud said in a statement.

Earlier this month, al Shabaab fighters killed 148 people on a university campus in the Kenyan town of Garissa, about 200 km (120 miles) from the Somali border.

The group has said it wants to punish Kenya for sending troops to Somalia as part of the African Union force. (Additional reporting by Feisal Omar and Abdi Sheikh in Mogadishu,; Writing by Drazen Jorgic; Editing by George Obulutsa and Gareth Jones)