The United Nations Children's Fund (UNICEF) confirmed Tuesday that two frontline workers were killed in a militant attack in Mogadishu.

The UN said the polio workers risked their lives to provide critical health services to vulnerable Somali children and offered condolences to the families.

The UN noted attacks on health workers are a violation of international law and must not be tolerated and urged authorities to conduct a comprehensive investigation, "Humanitarian workers should never be a target," it said.

Somalia is among high-risk polio outbreak countries because of its fragile and vulnerable population that mostly includes nomads, displaced people and those living in rural and slum areas.

No group has yet claimed responsibility for the attack but Somali-based, al-Qaeda-affiliated militant group, al-Shabaab, has claimed responsibility in the past for similar attacks in the Horn of Africa country.