At least four people have been killed and 15 others wounded in a suicide car bomb attack claimed by the al-Shabab group near the capital of Somalia, according to authorities.

Both Turkish and Somali officials said those injured in Saturday's attack near the town of Afgoye, about 30 kilometres (18 miles) southwest of Mogadishu, included Turkish engineers as well as Somali nationals working on a road in the area.

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Turkey's Health Minister Fahrettin Koca said six Turkish nationals and nine Somali citizens were wounded in the bombing, with two in critical condition and undergoing surgery.

The AFP news agency said at least four people were killed, citing local police officer Abdirahman Adan.

"A speeding suicide car bomb rammed into a place where the Turkish engineers and Somali police were having lunch," a different police officer, Nur Ali, told the Reuters News Agency.

Somalia-based al-Shabab armed group, which has stepped up activity in the East African country and neighbouring Kenya in recent weeks, claimed responsibility for the attack on their media outlet, Radio Andalus.

"We are behind the martyrdom of the suicide car bomb in Afgoye," said Abdiasis Abu Musab, spokesman for the group.

"We targeted the Turkish men and the Somali forces with them. There are casualties of death and injuries."

'The blast was huge'

Local residents described a massive explosion followed by "clouds of smoke".

"Before the blast, several Turkish engineers and well-armed convoy of Somali police were at the scene," Farah Abdullahi, a shopkeeper, told Reuters from Afgoye. "We see casualties being carried but we cannot make if they are dead or injured."

Another witness, Muhidin Yusuf, told the AFP news agency: "The blast was huge, it destroyed a container used by the Turkish engineers who work on the Afgoye road construction."

Turkey has been a significant source of aid to Somalia following a famine in 2011 as Ankara seeks to increase its influence in the Horn of Africa to counter Gulf rivals such as Saudi Arabia and the United Arab Emirates (UAE).

Turkish engineers have been helping with road construction in the country in recent years.

On Twitter, the Turkish Ministry of National Defense decried the attack. "We curse and condemn in the strongest terms the bomb terror attack which targeted innocent civilians in Somalia," the ministry said.

This is the latest in a string of attacks by the al-Qaeda-linked group since the beginning of the year. The group has been fighting for supremacy in the Horn of Africa country for years.

Al-Shabab controls large areas in the south and centre of Somalia and repeatedly attacks security forces and civilians there and in neighbouring Kenya.

In December, a group of Turkish engineers was among those hit in a blast at a checkpoint in Mogadishu that killed at least 78 people.

On January 5, al-Shabab stormed a military base used by US forces in Kenya's coastal Lamu region, killing three Americans.

Last week, the group warned Kenya will "never be safe", threatening tourists and calling for more attacks on US interests.

Kenyan and US forces have assisted the Somali government in its fight against the armed group.