Nairobi - A policeman was killed on Thursday after suspected Islamic extremists attacked a police station in the northern Kenyan county of Mandera, officials said, raising concerns over security in next week's national elections.Al-Shabaab militants are suspected in the attack on Lafey police station near the border with Somalia, North Eastern regional coordinator Mohamud Saleh said. Two vehicles were burnt in the early morning attack. The incident comes a day after three people died in a suspected al-Shabaab attack in southern Kenya and days before Tuesday's national elections, which al-Shabaab has threatened to disrupt.Former US Marine and security analyst Andrew Franklin said al-Shabaab has been threatening since March to disrupt the elections and they repeated the threat last week.The attacks are coming at time when security agents in the country are stretched preventing election violence, Franklin said."Without effective security the credibility of the elections cannot be assured," Franklin said.Al-Shabaab in recent weeks has stepped up deadly attacks in Kenya's border counties of Lamu and Mandera. Franklin said voting will be affected in those areas.The group has carried out more than 100 attacks inside Kenya, calling it retribution for the East African nation's deployment of troops to Somalia in 2011 to fight the extremists.