Nairobi - Eight Kenyan police officers were killed when their vehicles ran over improvised bombs in two separate incidents near the border with Somalia, officials said on Wednesday. The dead included a personal bodyguard for a local governor whose convoy was targeted.The deaths came a day after Kenya's police chief Joseph Boinnet announced that al-Shabaab extremists based in Somalia were stepping up attacks inside the country. He said al-Shabaab is under pressure from African Union troops supporting Somalia's government, which recently declared a new offensive against the extremist group.In the first incident, a police vehicle heading toward the border town of Liboi was blown apart by an improvised explosive device, North Eastern Regional Coordinator Mohamud Saleh said.Later on Wednesday, the governor of Mandera County, Ali Roba, said in a Facebook posting that five members of his security detail were killed when their vehicle was struck by a bomb. The dead included his personal bodyguard.Two other officers were injured in the attacks.Al-Shabaab, which has ties to al-Qaeda, claimed responsibility for the first attack in a report by its news agency, saying it had killed at least five Kenyan police officers, according to the SITE Intelligence Group, which monitors extremists.In the last two weeks, attacks by al-Shabaab in Kenya's Garissa and Mandera counties have increased after a lull. Last week, an improvised bomb killed four people in a vehicle, including a child.Mandera County has been hardest hit in recent years by an al-Shabaab campaign to avenge Kenyan troop presence in Somalia since 2011. Kenya's troops are part of the AU force there.Kenya has managed to stop the frequency of al-Shabaab attacks in its capital, Nairobi, and major towns, but human rights groups say the government uses methods such as extrajudicial killings that can fuel revenge attacks.