At least 27 members of Iran's elite Revolutionary Guards were killed and 10 wounded in a suicide attack the southeast of the country, Iranian news agency Fars reported on February 13.
The official government agency IRNA put the number of those killed at 20.
The militant Sunni Muslim separatist group Jaish al-Adl claimed responsibility for the attack.
The suicide bomber struck a bus transporting Revolutionary Guards members on the road between the cities of Zahedan and Khash, in the Sistan-Baluchistan Province, the Guards said in a statement carried by state media.
Sistan-Baluchistan is a volatile area near the border with Pakistan where militant groups and drug smugglers frequently operate.
A senior IRGC commander said in reaction to the attack that Iran's response will not be limited to Iranian borders.
Iranian authorities say Jaish al Adl operates from safe havens in Pakistan and Iran has repeatedly called on the neighboring country to crack down on them.
Revolutionary Guards commander Ali Fadavi was quoted as saying by Fars news agency "our response in defense of the Islamic Revolution will not be limited to our borders... The enemies will receive a very firm response from the Revolutionary Guards like before."
Last month Jaish al-Adl claimed responsibility for a double bombing in Zahedan, the provincial capital. Three police officers were wounded in the January 29 attack.
In December, a suicide bombing in the port city of Chabahar, also in the province, killed two police officers and wounded 42 people.
The province is populated mainly by ethnic Baluchis, who are Sunni Muslims. The region has also been plagued by violence by drugs traffickers.
There have been other attacks in recent days. On February 9, two police officers were killed when assailants opened fire on their vehicle. Later, police announced that nine suspects have been arrested. Days earlier another attack in western Iran claimed the life of a serviceman.