Twenty-seven members of Iran's elite Revolutionary Guard were killed in a suicide attack in the southeast of the country on Wednesday.
At least 10 members of the Guard were also wounded in the suicide car-bomb attack, which targeted a bus transporting the security force, Iran's Fars news reported.
"This incident will not damage the resolute will of the people in defending the Islamic revolution, and the national resolve for an unrelenting battle against terrorism will become more resolute than ever," said Iranian Vice President Eshaq Jahangiri, according to the Iranian Students' News Agency.
The state-run IRNA news agency, citing what it described as an "informed source", reported the attack on the security forces occurred in Iran's Sistan-Baluchestan province.
The province, which lies on a major opium trafficking route, has seen occasional clashes between Iranian forces and Baluch separatists, as well as drug traffickers.
The attack happened on the road between the cities of Zahedan and Khash, a volatile area near the Pakistan border where armed groups and drug smugglers frequently operate.
The Jaish al-Adl (Army of Justice) armed group claimed responsibility for the attack, according to Fars.
Images released by the news agency showed a hulk of twisted metal at the scene.
The Guard issued a statement confirming the attack. It said the troops were returning from the border.
"In this suicide attack a car filled with explosives blew up beside a bus carrying a unit of the Guard ground forces causing the martyrdom and wounding of a number of the protectors of our Islamic homeland's border," it said.
Foreign ministry spokesman Bahram Qassemi said security forces "will take revenge for the blood of the martyrs".
The attack came on the day of a US-led conference in Warsaw that included discussions on what the United States describes as Iran's "malign influence" across the wider Middle East.
Iranian officials have blamed Saudi Arabia and the United States for stirring up dissent in the country.
The Guard is a major economic and military power in Iran, answerable only to the country's supreme leader, Ayatollah Ali Khamenei.
In 2009, more than 40 people, including six Guard commanders, were killed in a suicide attack in Sistan-Baluchestan province. Jundallah, a pro-Sunni group still active in the region on Iran's border with Pakistan, claimed responsibility for that attack.
Most recently, an attack on a military parade in September 2018 in Iran's oil-rich southwest killed more than 20 and wounded about 60.