A bomb blast hit a bus carrying policemen in the island kingdom of Bahrain on Sunday, the interior ministry reported on its official Twitter account, wounding four officers.
"Terrorist blast in police bus near Jua village. 4 policemen injured and they are in a stable condition," the ministry wrote.
Anger among Bahrain 's mostly Shi'ite Muslim opposition against the Sunni-ruled monarchy mounted this year after three men convicted of a deadly 2014 bomb blast against policemen were executed in January.
Two policemen were killed in separate militant attacks the same month and security forces said this month they killed three fugitives in a gunbattle at sea as they tried to flee to Iran.
Shi'ite militant groups have launched sporadic bombing and shooting attacks against security forces since the government cracked down on 2011 "Arab Spring" protests in the kingdom with the help of neighboring Gulf Arab monarchies.
The opposition says it seeks to achieve more representation in government by peaceful means but the government accuses it of helping foment violence with help from regional arch rival Iran.