Two police officers were wounded Saturday in a blast, possibly caused by an improvised explosive device, in Kenya\'s Mandera region on the border with Somalia, police said. \"The officers were sheltering under a tree when the explosion went off, wounding both of them,\" said a senior police officer in Mandera. The two were taken to a hospital. Violent incidents involving local tribes, who often have automatic weapons, are frequent in northeast Kenya, but the number of explosions and hand-grenade attacks surged after the Kenyan army began hunting down al-Qaida-linked Islamist Shebab rebels in southern Somalia in late 2011. The U.S. embassy in Kenya earlier Saturday warned of the threat of an imminent attack on Mombasa and urged nationals to shun the country\'s second city, some 1,000 kilometers south of Mandera. The warning came as Kenyan police said they had detained two Iranian nationals over suspected links to a terror network planning bombings in Nairobi and Mombasa. In 1998 a series of truck-bomb explosions in Nairobi and Dar es Salaam in Tanzania killed hundreds of people. The date marked the eighth anniversary of the arrival of U.S. troops in Saudi Arabia in the build-up to the Gulf War.