An explosion near the world's largest refugee camp in Kenya has killed at least one policeman, police say.Two other officers were wounded when a suspected roadside bomb detonated near the Dadaab camp, Reuters news agency reports.The camp houses about 450,000 people who have fled famine and conflict in Somalia.Kenya has sent troops to Somalia to pursue militant Islamists it blames for a recent wave of abductions.The Islamist group al-Shabab said the Kenyan incursion was an act of war and it would take revenge.'Vehicle destroyed'Dadaab deputy police chief Nelson Kaliti said one policeman was killed in the blast, Reuters says.A private security officer, who declined to be named, said two other policemen were wounded, it reports."We heard an explosion and on our arrival saw the vehicle ripped in two. The front part of the wreckage was lying on top of the mutilated body of the driver," the officer is quoted as saying."Two other badly wounded policemen were pulled out of the wreckage," he said.The BBC's Bashkas Jugsodaay in the capital, Nairobi, says Dadaab residents told him there is a heavy security force presence at the camp, and police have even sealed off the hospital.Two Spanish women working for the medical charity Medecins Sans Frontieres (MSF) in Dadaab were kidnapped in October.Somalia has not had a functioning central government for more than 20 years and has been wracked by fighting.Three Somali areas are currently suffering famine conditions, the UN says.