Al-Shabab is an armed group based in Somalia, fighting to topple the Western-backed Somali government. It wants to impose a strict version of Islamic law in the Horn of Africa country.

The armed group has carried out several attacks in neighbouring countries, including Kenya, since Nairobi launched a military operation inside Somalia in 2011.

The group used to control most parts of south and central Somalia, but since 2010, the fighters have been expelled from most major towns as the US military and a 20,000-strong African Union peacekeeping mission boosted the Somali forces.

Below we take a look at the major attacks carried out by the group in Kenya

2013: In September 2013, al-Shabab fighters stormed Nairobi's Westgate Mall, firing indiscriminately at shoppers and killing 67 people in a siege that lasted 80 hours.

The assault horrified the world and exposed weaknesses in Kenya's security forces.

2014: In November 2014, members of Somalia's al-Shabab armed group hijacked a bus in Kenya and killed 28 non-Muslims on board.

"What happened in Mandera today we did in revenge for what the non-believer government has done to innocent Muslims," Sheikh Ali Mohamud Rage, a spokesman for the group, told Al Jazeera.

April 2015: Al-Shabab launched an assault on Garissa University College in Kenya, killing 148, mainly students.

January 2016: Fighters from the Somali armed group assaulted a Kenyan-run military base for African Union peacekeepers, killing scores of Kenyan soldiers.

January 2019: Two explosions and gunfire heard at an upscale hotel complex in Nairobi, Kenya's capital.

The attack on the Dusit hotel complex - which also houses offices and banks - sent people fleeing for their lives.

There could still be armed assailants in the building and the police operation was ongoing, reporters were told in a short briefing.