A police officer was killed in clashes with militants in the south-east of Iran early on Wednesday, the official Islamic Republic News Agency said.
Gunmen attacked a checkpoint in Rask County in the province of Sistan and Baluchestan at about 1.30am local time, Irna reported.
The clashes lasted about three hours before the arrival of police reinforcements prompted the attackers to withdraw, it said.
"In an exchange of fire this morning between forces from a police station in Rask and members of an armed group, one of the policemen was killed," it added.
Sistan and Baluchestan is one of the few majority-Sunni provinces in Shiite-majority Iran and has been plagued by persistent unrest involving drug-smuggling gangs and rebels from the Baluchi ethnic minority, as well as extremist groups.
The attack was claimed by the hardline militant group Jaish Al Adl (Army of Justice) in a brief statement on its Telegram channel, according to AFP.
Jaish Al Adl was formed in 2012 and is blacklisted by Iran as a terrorist group.
This was the second attack by gunmen in a month against police in the province after one in mid-December in which eleven police officers were killed and seven wounded, Irna said.
Last week, Iran's Interior Minister Ahmad Vahidi announced new measures to firm up security along the porous border, following twin suicide bombings claimed by ISIS that killed more than 90 people at a high-profile state event in the southern city of Kerman.
Mr Vahidi said authorities had identified "priority points to block along the border".
On Wednesday, he confirmed one of the suicide bombers was a Tajik, and the "second one was most likely also Tajik", Irna reported.
ISIS draws many recruits from the Tajik ethnic group, who live primarily in Afghanistan, Tajikistan and Uzbekistan. The attacks on Kerman were claimed by the branch of ISIS based in Afghanistan.
Many Tajiks were active fighters and field leaders when the terrorist group controlled large areas in Iraq from mid-2014 to late 2017.
