Nairobi - Suspected al-Shabaab extremists attacked vehicles in Kenya's coastal county of Lamu and killed three people just days before national elections are held, officials said Wednesday.A bus was traveling from Mombasa to Kipini in the afternoon when fighters shot at it from the bushes, coast regional police boss Larry Kieng said. He said the fighters fled once police escorting the bus engaged them. Five people in the bus were wounded.A private vehicle was later attacked on the same road and set on fire, killing its three passengers, said Joseph Kanyiri, who is in charge of a multi-agency task force to clear al-Shabaab fighters from a dense forest in the area they have been using as an operating base. Two motorbikes were also set on fire.Al-Shabaab in recent weeks has stepped up deadly attacks in Kenya's border counties of Lamu and Mandera. The extremist group based in neighboring Somalia has threatened to disrupt Kenya's elections on Tuesday.Al-Shabaab has carried out more than 100 attacks inside Kenya, calling it retribution for the East African nation deploying troops to Somalia in 2011 to fight the extremists.Police escorts for buses travelling through Lamu and Mandera have been mandatory since November 2014 after al-Shabaab extremists stopped a bus heading to Mandera and killed 28 passengers after separating them from Muslims.