US Vice President Joe Biden on Monday sharply criticized the Islamist Shebab rebels over Somalia's famine, saying that the group has hindered efforts to bring food to the hungry.

The United Nations estimates that 3.7 million Somalis -- around one-third of the population -- are on the brink of starvation and tens of thousands have already died in a country that has lacked a government for two decades.

"Al-Shebab terrorists did not create the food crisis but they have made it far worse. Drought conditions exist throughout East Africa but so far famine is concentrated only in the Al-Shebab-controlled areas," Biden told a forum on global hunger held at the State Department.

"The most cynical action of all, they endanger their own people by commandeering assistance sent by the rest of the world to the starving children and women of that country," he said.

Shebab, which takes inspiration from Al-Qaeda, has banned Western influence in its regions and has been accused of threatening and kidnapping aid workers, including two Spanish women snatched working with refugees in Kenya.

Kenya has launched an unprecedented week-old military push into Somalia against the Shebab, which threatened retaliation. One person was killed and 29 were wounded in two grenade attacks on Monday in Kenya's capital Nairobi.

In an apparent publicity push, Al-Qaeda recently held a ceremony where the militants were seen donating food, money and clothes.

"Make no mistake -- it's not that Al-Shebab cares that much about innocent people dying. Rather, they're concerned that these grim conditions threaten their grip on the region and undermine their propaganda purporting to defend the Somali people," Biden said.