LONDON (Reuters) - Three Iranian security personnel were killed by militants in a cross-border attack on the frontier with Pakistan on Tuesday, Iran’s Revolutionary Guards said.

An Iranian police officer was killed in an ambush on a border post in the city of Mirjaveh in Sistan-Baluchestan province, and two soldiers died when their vehicle was hit by an improvised explosive device (IED), the Guards said.

The Revolutionary Guards’ statement, posted on state media, said three people it described as terrorists were killed by Iranian security in a “firm and timely response”.

Sistan-Baluchestan province in southeastern Iran has long been plagued by unrest from both drug smuggling gangs and separatist militants. The population of the province is predominantly Sunni Muslim; the majority of Iranians are Shi’ites.

In April 2017, ten Iranian border guards were killed by militants in Mirjaveh.

Sunni militant groups which have carried out several deadly attacks in recent years say they are fighting against discrimination.