The group, now totally controlled by hard-line jihadists after an internal power struggle, is striking out against Somalia's Western-backed transitional government and seems determined to take its 8-year-old conflict to East African countries that support Mogadishu.
Sunday's airstrike underlined U.S. concerns about al-Shabaab's ability to mount operations, including Kenya and Ethiopia, U.S. allies which have contributed substantial numbers of troops to the 18,000-strong African Union peacekeeping force, known as Amisom, which is vital to the survival of the Mogadishu government.
U.S. officials acknowledged the strike on the village of Hawai in the Lower Shabelle region of southern Somalia was aimed at killing a top al-Shabaab figure, but declined to identify the target.
However, Somalia sources said the strike killed Sahal Iskudhuq, a top-tier commander and a high-ranking official in al-Shabaab's highly secretive intelligence unit, the Amniyat, and was close to its overall leader, Ahmed Abdi Godane.
U.S. special forces have killed several al-Shabaab leaders in recent years and in October 2013 attempted to capture Abdulkadir Mohammed Abdulkadir, a Kenyan of Somali origin known as " Ikrima" in an abortive amphibious operation.
In October 2013, the U.S. displayed its concerns about al-Shabaab by secretly deploying a small unit of 5 to 10 advisers to Mogadishu to help the government coordinate operations against al-Shabaab.
They were the first U.S. military personnel based there since the infamous Black Hawk Down episode in 1993 in which 18 American troops were killed.
Iskudhuq's death would be a severe blow to the organization and to Godane, who declared al-Shabaab's allegiance to al-Qaida in 2011.
Senior al-Qaida operatives have served as al-Shabaab leaders in recent years, including Fazul Abdullah Mohammed, mastermind of the August 1998 bombings of the U.S. embassies in Nairobi and Dar es Salaam killed by Somali troops at a Mogadishu checkpoint in June 2011.
Kenya, Somalia's northern neighbor, which has been repeatedly targeted by al-Shabaab for aiding Mogadishu, claimed Jan. 9 it killed 30 militants, including three senior leaders, in an airstrike on the group's Birta Dhere camp in the Gedo region near the Ethiopian border.
Kenya has stepped up operations against al-Shabaab since Somali jihadists seized the upscale Westgate shopping mall in central Nairobi, Kenya's capital, in September 2013, their boldest strike outside Somalia since 2010 bombings in Kampala, Uganda, that killed 74 people.
At least 67 people were killed in the Nairobi assault, including four al-Shabaab infiltrators. Nairobi was targeted because of the 4,000 troops it has deployed with Amisom and its crackdown on al-Shabaab sympathizers in Kenya's large Somali community, many of them refugees.
The Birta Dhere strike was launched on the basis of intelligence gathered by Kenya's military that al-Shabaab leaders, including Ahmed Abdi Godane, were meeting in the camp, Western intelligence officials said. If Godane was there, he apparently got away.
Godane, aka Mukhtar Abu Zubayr, seized control of al-Shabaab in June 2013 when he came out on top of a long-running power struggle with his main rival, the group's spiritual leader and elder statesman, Sheik Hassan Dahir Aweys.
Godane, who trained in Afghanistan with al-Qaida, executed Ibrahim al-Afghani, one of the group's co-founders, who was close to Osama bin Laden. Aweys fled and was subsequently captured by government forces.
Godane favors transnational holy war. He has close links with al-Qaida leader Ayman al-Zawahiri, the veteran Egyptian Islamist who took over al-Qaida Central in northern Pakistan after bin Laden was assassinated by U.S. forces near Islamabad May 2, 2011.
Aweys, a cleric, espouses a narrower, more nationalist Somali agenda and opposes foreign intervention in Somalia, which has been torn by clan warfare since dictator Mohammed Said Barre was overthrown in 1991.
Despite the loss of most of al-Shabaab's urban strongholds, particularly the port of Kismayo, to Amison and African forces in 2011-12, the group still holds large swathes of central and southern Somalia.
"The apparent decision by Godane and fellow hard-liners to again take the fight beyond Somalia's borders looks like a bid to regain the initiative in the face of the recent setbacks and disagreements," observed British analyst Simon Tidsall.