Four people were killed in a car bombing in Somalia on Saturday that apparently targeted Turkish engineers working on a road near the capital Mogadishu, police and witnesses said.

The attack was claimed by the al-Qaeda-linked extremist militant group al-Shabaab, which has stepped up its activities in Somalia and neighboring Kenya in recent weeks.

The bomb struck near the town of Afgoye about 30 kilometers (20 miles) west of the capital, killing four people and wounding several others including several Turkish nationals, said local police officer Abdirahman Adan.

“The blast was huge, it destroyed a container used by the Turkish engineers who work on the Afgoye road construction,” said witness Muhidin Yusuf.

“There were police who were guarding the Turkish engineers and several other people gathering near the checkpoint where the temporary shelter is located,” said another witness Ahmed Said.

“I saw the dead bodies of several (people) and Turkish workers who were wounded in the blast.”

The extremist group, which has fought for more than a decade to topple the Somali government, has carried out a series of attacks in recent weeks including a massive car bombing in Mogadishu on December 28 that killed 81 people.

And on January 5, the extremists stormed a military base used by US forces in Kenya’s coastal Lamu region, killing three Americans.

Last week, al-Shabaab warned that Kenya will “never be safe”, threatening tourists and calling for more attacks on US interests.

Kenya sent troops into Somalia in 2011 as part of an African Union peacekeeping mission fighting against al-Shabaab, and has seen several brutal retaliation attacks both on its troops in Somalia and civilians in Kenya.

Earlier reports said that at least two people were killed and more than 20 others wounded when a suicide car bomber targeted a construction site along a highway outside Somalia’s capital.

Six Turkish nationals were among the wounded, with two in serious condition, Turkish Health Minister Fahrettin Koca said.

The Turkish construction workers appeared to be the bomber’s target, Somali police Col. Abdi Abdullahi said.

Turkey has invested heavily in Somalia, with technical and development assistance exceeding $1 billion, according to the Turkish government. Turkish companies run the international airport and seaport in Mogadishu, and in 2016 the Turkish president inaugurated Turkey’s largest embassy complex in the world there.