Three Syrian doctors and their Somali colleague have been shot dead in an ambush near Somalia's capital, Mogadishu, witnesses have told the BBC.
Gunmen opened fire on their car as they travelled towards a health clinic, the eyewitnesses said.
Two of their bodyguards were also killed in the attack, while another Syrian and Somali doctor were seriously wounded, they said.
Many militias and criminal gangs operate in Somalia.
In recent years, the country has also been hit by an insurgency waged by the al-Qaeda-linked al-Shabab group.
However, it did not carry out the attack, an unnamed senior official of the group told the AFP news agency.
The Syrian doctors were believed to be in Somalia as aid workers.
The BBC's Mohamed Moalimu reports from Mogadishu that the ambush took place in Siinka Dheer, a government-controlled district about 20km (13 miles) south of the capital.
The gunmen escaped after firing indiscriminately at the vehicle, eyewitnesses told him.
The two wounded doctors are receiving emergency treatment at Mogadishu's Medina Hospital, our reporter adds.
The bodies of the six people killed in the attack have also been brought to the hospital, he says.
In August, medical charity Medecins Sans Frontieres (MSF) announced that it was closing all its programmes in Somalia because of "extreme attacks on its staff".
Armed groups and civilian leaders increasingly "support, tolerate or condone the killing, assaulting, and abducting of humanitarian aid workers", it said at the time.
MSF had worked for 22 years in the war-torn country.
Over the last two years, al-Shabab militants have been driven out of Somalia's major towns and cities by pro-government forces and a UN-mandated African Union force of some 18,000 soldiers.
But the group still controls many towns and rural areas of southern Somalia.
It says it is fighting to establish an Islamic state in Somalia.
Reproduced under licence from BBC News © 2013 BBC