Four foreign aid workers and one Kenyan national have been kidnapped from a refugee camp in Kenya close to the border with Somalia, police say.Sources have told the BBC that a convoy of aid workers from the Norwegian Refugee Council was ambushed by gunmen.Police sources told AFP agency that a driver was killed in the assault in Dadaab camp.Several aid workers have been kidnapped from Dadaab in the last year and many groups have withdrawn from the camp.Philip Ndolo, of the Kenyan police, said two vehicles in the convoy had come under attack - and one had managed to get away.A spokesman for the Norwegian Refugee Council said its secretary general, Elizabeth Rasmussen, was in the convoy that was involved in an "incident", but she was fine.No further details were available, he said.Somalia has had no effective central government since 1991, and has been wracked by fighting ever since - a situation that has allowed piracy and lawlessness to flourish.Islamists from the al-Qaeda-linked group al-Shabab control large swathes of the country."So far we have not confirmed the nationalities of the four foreign workers who were kidnapped," Mr Ndolo told Reuters news agency."We suspect this could be the work of al-Shabab sympathisers."Last October, Kenyan troops entered Somalia in pursuit of al-Shabab militants they accuse of being behind various kidnappings on Kenyan soil and of destabilising the border region.Earlier that month two Spanish doctors working for the medical charity Medecins Sans Frontieres were kidnapped from Dadaab and are still being held hostage.