At least three people were killed Wednesday when gunmen sprayed a passenger bus with bullets in northeastern Kenya, police said, the latest attack by suspected al-Shabaab militants from neighbouring Somalia. "We had an incident of a bus attack and there are three fatalities," said a senior police officer in Mandera, a county along the border with Somalia. The bus owner said the assault occurred around 11:00 when armed men opened fire on the bus after it left Mandera, a city which shares the name of the wider county. He said the bus did not have a security escort, a precaution sometimes taken in a region where attacks by al-Shabaab Islamists are frequent.Since the start of the year, the al-Qaeda linked militants have stepped up attacks in eastern Kenya, along the Somali border.Last month three people were killed near the country's southern border with Somalia when gunmen ambushed a bus they were travelling in. Al-Shabaab was also suspected in an attack south of Garissa, another border region, that left three teachers dead.On January 5, the Islamists stormed onto a US military base in the coastal Lamu region, destroying several aircraft and killing three Americans.Two days later they killed four civilians, including a child, during an attack on a telecommunications mast near Garissa.Al-Shabaab issued a statement in January warning that Kenya "will never be safe", threatening tourists and US interests in the country.The group said Kenya should withdraw its forces from Somalia while they still "have the chance".Kenya sent troops into Somalia in 2011 as part of an African Union peacekeeping mission fighting against Al-Shabaab, and has seen several brutal retaliation attacks both on its troops in Somalia and civilians in Kenya.