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 Shia 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.
Shia 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 neighbouring Gulf Arab monarchies.