Iran's Foreign Ministry spokesman Bahram Qassemi made the condemnation on Saturday while offering his condolences to the Syrian government and nation.
Earlier, at least 40 people were killed in the blast, which hit al-Rashideen district on the western outskirts of Aleppo, located some 355 kilometers north of the capital Damascus, as buses were stopping at a checkpoint.
The buses were evacuating residents of Kefraya and al-Foua villages in accordance with an agreement reached between the Syrian government and foreign-sponsored Takfiri militant groups last month.
"The shameful criminal act by Takfiri terrorists in the cowardly attack on buses carrying the people of al-Foua and Kefraya in Syria and killing tens of innocent and defenseless women and children have added to the record of atrocities carried out by terrorists and their supporters,” said Qassemi.
He went on to slam those who claim to be supporters of the Syrian nation but refrain from commenting on such actions.
Qassemi further stressed that dividing militants into "good and bad” by supporters of such groups only serves to embolden terrorists, Press TV reported.
Evacuee buses arrive in Aleppo
Later on Saturday, the SANA news agency announced that the final series of buses taking people from the villages of al-Foua and Kefraya have arrived in Aleppo.
Some 15 buses transporting hundreds of civilians arrived in al-Ramousa where they were put up in a fully stocked temporary housing center.
Before that, several ambulances transporting those injured in the earlier attacks arrived in the city.
A total of 75 buses and 20 ambulances had been scheduled to arrive in Aleppo on Friday, but infighting between the terrorists had halted the convoy’s departure.