Three people have been killed after a bus was attacked in Kenya close to the Somali border, local officials say.Two others were injured and are being treated at local clinics.The bus was travelling from Mombasa and was targeted in Lamu county when heavily armed men stopped it and sprayed it with bullets.The gunmen have not been identified, but the Somalia-based al-Shabab militant group has carried out a number of attacks in the country.Local authorities say security forces are pursuing the attackers, who fled the scene.Kenya has troops in Somalia supporting the UN-backed government in its fight against al-Shabab.The country has been on high alert over the Christmas and New Year holiday period, following intelligence reports that al-Shabab could carry out attacks.