Kenya's security officers in the coastal city of Mombasa are on high alert after an improvised explosive device was found at a local school on Wednesday. Likoni divisional police commander Robert Mureithi said unknown people hurled an explosive device at a primary school in Likoni area of Mombasa which exploded inside the school but did not cause injuries. "Lucky enough, there were not injured and a team of bomb experts are at the scene to establish the type of the device which was used," Mureithi said. The port city of Mombasa, the capital Nairobi and other parts of Kenya have suffered a series of grenade attacks since Kenya sent troops into Somalia last year to try to pursue Al-Shabaab insurgents it blames for a surge in violence and kidnappings threatening tourism. Kenya's tourism has suffered a decline the number of tourists arriving since September 2011 when the Somali militant group, the Al-Shabaab, carried out the kidnappings of tourists in the Lamu archipelago and the kidnapping of the Spanish volunteers. Several people have been killed and hundreds injured. Most of the attacks have been in Kenya's northern region near the Somali border, but some have been in Mombasa and Nairobi.