(Adds background) MANAMA, March 25 (Reuters) - A blast caused by a homemade bomb on a highway west of Bahrain's capital Manama caused mi**r injuries to several policemen, the Gulf Arab kingdom's Interior Ministry said on Tuesday. Bahrain has experienced unrest over the past three years by majority Shi'ite Muslims demanding political reform and an end to what they see as sectarian discrimination, something the government denies. Bahrain is closely watched by Western countries because it is home to the U.S. Fifth Fleet and has become a frontline in regional tensions between Shi'ite Iran and Sunni Saudi Arabia. Bahrain has accused Iran of fomenting unrest in the kingdom.