At least 27 members of Iran's Revolutionary Guards were killed in a suicide bombing on Wednesday in southeastern Iran.
At least 13 others were injured in the attack in Sistan and Baluchistan province-Iran's second-largest.
In a statement, the Islamic Revolutionary Guard Corps (IRGC) said a bomb-laden vehicle targeted a bus carrying personnel.
The Jaish al-Adl ("Army of Justice"), which claims to support Sunni rights in Shia Iran, claimed responsibility for the attack.
Iranian government officials have yet to make a statement.
Meanwhile, Iran's Foreign Minister Javad Zarif took Twitter: "Is it no coincidence that Iran is hit by terror on the very day that #WarsawCircus begins?"
"Especially when cohorts of same terrorists cheer it from Warsaw streets & support it with Twitter bots? US seems to always make the same wrong choices, but expect different results," Zarif added.
Representatives of several Arab and Western states attended a U.S.-sponsored Mideast conference held in Warsaw on Feb. 13-14. The international ministerial meeting will discuss peace and security in the Middle East, and critical issues such as terrorism, missile development and proliferation.
Separately, a Sunni cleric in Iran, Molavi Abdolhamid Ismaeelzahi condemned the attack.
Ismaeelzahi said people of Sistan and Baluchistan were suffering from such attacks: "The only victims of [such attacks] are our people who want to live in peace and security."
Meanwhile, the governorship of the province declared three-day period of mourning after the attack.
Last September an attack on a military parade in Ahvaz, in southwestern Iran killed 25 people -- including Revolutionary Guard members -- and wounded dozens, in one of the deadliest attacks in the country in years.