MOGADISHU, Somalia -- A bomb explosion on a bus carrying U.N. employees in northern Somalia killed at least six people and wounded many others Monday, police and U.N. officials said.

Col. Ali Salad, a senior police officer in the semiautonomous Puntland region, said by phone that the victims of the attack in the town of Garowe, the capital of Puntland, included both foreigners and Somalis.

"It's a dark day, but terrorists must know that the blood they shed will not go in vain. We shall deal with them with an iron hand," he said.

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The U.N. representative to Somalia, Nicholas Kay, condemned the attack in a Twitter post, saying he was "shocked and appalled by loss of life."

According to the Reuters news agency, the attack hit staffers with the United Nations Children's Fund (UNICEF). Reuters quoted a statement from the agency saying the "attack occurred when the staff were traveling from their guest house to the office, normally a three minute drive."

The al Qaeda-linked al-Shabaab group claimed responsibility for the attack, according to the group's Andalus radio.

The bomb was apparently planted under a seat and was detonated by remote control, said police official Yusuf Ali.

Garowe resident Jama Hashi said he heard a thundering blast inside the bus, which he said was passing near the offices of the U.N.'s food agency when the bomb went off. Human limbs were scattered around the scene of the attack, he said.

Security forces sealed off the area as ambulances carried the wounded from the scene.

Bomb attacks are not common in the northern parts of Somalia, unlike in the south where al-Shabaab militants are waging a deadly war against the Western-backed Somali government and the African Union forces bolstering it.

Last week at least 10 people were killed in an assault on the offices of Somalia's education ministry.

Despite losing a lot of ground in recent times and losing top leaders in airstrikes, al-Shabaab militants are still able to launch attacks in different parts of Somalia and even across the border, especially in Kenya.

Al-Shabaab claimed responsibility for an attack earlier this month at a university campus in northeast Kenya in which militants killed 148 people, most of them students.