A famine in Somalia in 2011 killed an estimated 260,000 people, half of them under the age of 6, according to a report to be published this week. The estimate is more than double previous assessments of the toll exacted by the famine. Many international aid activists believe that tens of thousands of people died needlessly because outside nations were slow to respond to early signs of approaching hunger in East Africa in late 2010 and early 2011. The toll was also exacerbated by extremist militants from the Shabab, who prevented food deliveries to the areas of south-central Somalia that they controlled. A previous estimate by the British government determined that between 50,000 and 100,000 people had died in the famine. Three international officials who had been briefed on the new report, to be made public on Thursday, confirmed its findings.