**Four Afghan policemen have been killed in a roadside bomb explosion in western Afghanistan, officials say.**The incident happened when a police vehicle hit a bomb in Rubat-i-Sangin district in Herat province on Tuesday night, officials added.
It comes two days after suspected militants killed at least 16 policemen in two attacks in Afghanistan.
Afghan police have often been targets of militants and have suffered much heavier casualties than foreign troops.
There has been no immediate claim of responsibility for the latest attack, but roadside bomb attacks on police in the past have been blamed on the Taliban.
On Monday, gunmen targeted police posts in Baghlan province in the north and in Helmand province in the south. In each ambush eight policemen were killed.
Correspondents say that the insurgents have targeted those regarded as working with the government.
Police have borne the brunt of losses - with hundreds killed in the past few years.