The al-Qaida-affiliated militant group al-Shabaab is in control of large swathes of southern and central Somalia and is gaining enough strength to possibly overthrow a weak, pro-Western regime in Mogadishu, counter-terrorism experts told ABC News.

"U.S. security interests are gravely threatened if a country again becomes what Afghanistan used to be: a safe haven for al-Qaida, a government run by al-Qaida affiliates, and a place where al-Qaida supporters from around the world can come safely and train to get ready to launch terrorist attacks around the world," former White House counter-terrorism adviser Richard Clarke told the U.S. broadcaster.

Al-Shabaab is also recruiting U.S. Somalis, many of whom live in Minneapolis, officials said. They told ABC a former Minneapolis resident and naturalized U.S. citizen, Shirwa Ahmed, is thought to have committed a suicide bombing in an attack last month in northern Somalia, and that more than a dozen young men from Minnesota may have gone to Somalia to join the militants.