Here is some background on the Al-Qaeda affiliate which has been fighting to overthrow the weak central government in Mogadishu for more than 15 years.
Al-Shabaab emerged from Somalia's Islamic Courts Union that controlled central and southern Somalia for six months in 2006 before being ousted by Ethiopian troops.
The group, which advocates a strict version of sharia or Islamic law, declared its allegiance to Al-Qaeda and was officially integrated in 2012. It was designated a "foreign terrorist organisation" by the US State Department in 2008.
Ahmed Diriye became the new "emir" after the killing of Ahmed Abdi Godane in a US strike in September 2014. Diriye, also known as Ahmed Umar and Abu Ubaidah, has a $6-million US bounty on his head.
Al-Shabaab fighters were chased out of their last bastions in Mogadishu in 2011 by a 22,000-strong African Union mission which has been in Somalia since 2007.
The following year they were routed from the port city of Kismayo but they retain control over vast rural areas and still launch attacks on military, political and civilian targets.
The deadly insurrection is the biggest security challenge for new President Hassan Sheikh Mohamud, who is also having to contend with a devastating drought that has left millions hungry and fractious relations with several regions.
The International Crisis Group said in a June analysis that Al-Shabaab had been emboldened by the Taliban's takeover in Afghanistan and suggested that Mohamud should "test the waters" and try to engage with the jihadists.
"Little suggests that Al-Shabaab will be defeated militarily but nor are militants likely to prevail in the long run."
Al-Shabaab is estimated to have between 7,000 and 12,000 fighters as of this year, according to the CIA World Factbook.
In the deadliest attack in Somalia to date, a truck bombing in a busy neighbourhood of Mogadishu in October 2017 killed more than 500 people.
Beyond its borders
Al-Shabaab has carried out a spate of bloody attacks in neighbouring Kenya since its troops intervened in Somalia in 2011 as part of the AU force then known as AMISOM.
The deadliest was in April 2015 when 148 people, mainly students, were killed at Garissa University in northeastern Kenya.
In September 2013, Al-Shabaab claimed a bloody siege on the Westgate shopping mall in Nairobi that killed 67 people over four days.
The following year, in June-July, around 100 people were killed in raids in the Lamu region on Kenya's Indian Ocean coast, home to the popular tourist island of the same name.
In January 2019, an hours-long attack on an upscale Nairobi hotel complex left at least 21 dead.
Its first major operation outside Somalia was in July 2010 in Uganda, the main contributor to the AU force. It carried out suicide bombings in two restaurants in the capital Kampala that killed 76 people.
In May 2014, it claimed an attack on a Djibouti restaurant packed with Westerners, saying it was targeting French "crusaders" and that the strike was in retaliation for Djibouti's hosting of the biggest US military base in Africa.
In the most recent action outside Somalia, Al-Shabaab fighters raided a military base on the border with Ethiopia in July, causing an unknown number of casualties.
It followed an incursion into Ethiopia, where local authorities in the Somali region said a three-day military operation had killed more than 100 Al-Shabaab members.
The United Nations voted in March to replace AMISOM with ATMIS (the African Union Transition Mission in Somalia) which has a more offensive remit than its predecessor.
Its goal is to gradually reduce troop numbers to zero by the end of 2024 with security to be assumed by Somalia's army and police, although analysts doubt they will be ready by then.
The force is drawn from Uganda, Burundi, Djibouti, Ethiopia and Kenya, with troops deployed in southern and central Somalia.
In May, President Joe Biden ordered the reestablishment of a US troop presence in Somalia, reversing a move by his predecessor Donald Trump, who in late 2020 pulled out nearly all US forces.
Fewer than 500 troops will be involved, slightly smaller than the original number of about 750 who had spent years in the country.
Since Biden's announcement, US forces have carried out a number of deadly air strikes against Al-Shabaab operatives in Somalia.