A lone gunman fired his rifle indiscriminately, wounding five other soldiers before he was gunned down, a Kenyan military official told the Associated Press (AP) news agency, requesting anonymity because he was not yet authorised to reveal the information.

Through its media, al-Shabab confirmed it had carried out the attack.

The attack on Monday morning took place at the Sarira Forward Operating Base in the Lower Jubba region of southern Somalia near the border with Kenya.

“We believe the lone wolf was testing the ground for more such incidents. We must be more careful and vigilant,” the unnamed Kenyan military official told the AP. “As we near the festivities, we need to be very vigilant of our surroundings. The terror threat is still rife and all measures should be taken to tame any plan.”

Kenyan authorities also are urging vigilance after the start of the football World Cup in Qatar. In 2010, at least 76 people were killed when al-Shabab went after a rugby club and a restaurant in the Ugandan capital, Kampala, where customers were watching the World Cup final on giant screens.

“It is at this time the militants know we might let our guard down as we watch” the World Cup, the official said.

Al-Shabab, an al-Qaeda-allied armed group fighting in Somalia for more than 10 years, is seeking to topple the country’s central government and establish its own rule based on a strict interpretation of Islamic law.

Its fighters were driven out of Mogadishu in 2011 by the African Union peacekeeping forces. But it still controls swathes of Somalia’s countryside and has stepped up attacks since President Hassan Sheikh Mohamud took office in May and pledged an “all-out war” against the group.

Kenyan troops, who are deployed to Somalia under the banner of the African Union, are active along the border with Somalia. Peacekeepers from Burundi and Uganda are based in or near the Somali capital of Mogadishu, the seat of the federal government.

In recent years, al-Shabab fighters have launched cross-border attacks, especially in the remote Kenyan counties of Mandera and Garissa, after breaching security zones. The attacks have killed or wounded dozens of civilians and security personnel.