At least 27 members of Iran's elite Islamic Revolutionary Guards Corps (IRGC) have been killed in a suicide attack in the southeast of the country, Iranian news agencies report.
The militant Sunni Muslim separatist group Jaish al-Adl claimed responsibility for the attack.
The suicide bomber struck a bus transporting IRGC members on the road between the cities of Zahedan and Khash in Sistan-Baluchistan Province, the IRGC said in a statement carried by state media.
Sistan-Baluchistan is a volatile province near the border with Pakistan, where militant groups and drug smugglers frequently operate.
Iranian Foreign Minister Mohammad Javad Zarif linked the February 13 attack to a Middle East conference taking place in Warsaw, jointly organized by the United States and Poland, which Tehran has labeled an anti-Iran gathering.
"Is it no coincidence that Iran is hit by terror on the very day that #WarsawCircus begins?" Zarif wrote on Twitter, claiming that people on the streets of Warsaw were cheering for the attackers.
He offered no evidence to support his claim.
The attack is a fresh blow to the image of the IRGC, the most powerful security force in Iran, which reports directly to Supreme Leader Ayatollah Ali Khamenei.
Twelve members of the IRGC were among 25 people killed last autumn in an attack by gunmen on a military parade in the city of Ahvaz. The Islamic State militant group and an Arab separatist faction claimed responsibility for that attack.
Jaish al Adl says it wants greater rights for the Baluchs.
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.
In October, Jaish al-Adl claimed responsibility for kidnapping 10 Iranian security personnel, some of whom have been released in the meantime.
With reporting by AP, Reuters, dpa, and AFP