WASHINGTON (AFP) - The United States promised Tuesday it would not prosecute aid groups that deal with Somalia's Al-Qaeda-linked Shebab in a desperate bid to stop a famine that has already killed thousands. US officials said Washington was maintaining sanctions against the militia, which controls some of the worst-hit parts of southern Somalia, but would show "flexibility" toward aid groups trying to bring in food. "Our number one goal is to save lives. Time is not on our side," a US official said on condition of anonymity. The United States imposed sanctions on the Shebab in 2008 which makes it a crime to provide any support to the militia, which has emerged as a major force in a country that has lacked a functioning government for two decades. US officials said the sanctions were never intended to affect aid groups, but that Washington wanted to send a signal that relief workers should not fear legal repercussions for bringing food where it is most needed. "What we're doing is working to reassure humanitarian assistance organizations and workers that good-faith efforts to deliver food to people in need will not risk prosecution," another US official said. The Shebab expelled foreign aid groups two years ago, accusing them of being Western spies and Christian crusaders. But the group has recently given mixed signals on whether the ban is in effect; some relief workers have said the militia has tried to impose a "tax" on food delivery. US officials said they did not anticipate a change in US laws, but that the Treasury Department has adopted new guidance as it enforces sanctions. Humanitarian groups have called the famine in the Horn of Africa the worst in six decades, brought on by severe drought -- likely linked to climate change -- and the virtual absence of any governance in Somalia. The UN humanitarian chief, Valeria Amos, said Monday that tens of thousands have already died in the Horn of Africa famine and said some 12.4 million people in Kenya, Ethiopia, Somalia and Djibouti are in dire need of help. Eritrea, an authoritarian state which the United States accuses of assisting the Shebab fighters, has reported no food shortages but US weather analyses indicate that it is also affected by the region's drought. About $1 billion has already been promised by the international community, but Amos said Monday the United Nations "urgently needs another $1.4 billion to save lives."