Iran's Revolutionary Guard Corps has accused Saudi Arabia of being behind Wednesday's double attack in the capital Tehran that left at least 13 people dead.
The Islamic State of Iraq and the Levant (ISIL, also known as ISIS) claimed responsibility and released a video purporting to show gunmen inside the parliament building.
The elite force vowed in a statement on Wednesday to seek revenge.
"The fact that Islamic State has claimed responsibility proves that they were involved in the brutal attack," referring to Saudi Arabia.
The statement also said that the "spilled blood of the innocent will not remain unavenged".
Saudi Foreign Minister Adel Al-Jubeir, speaking in the German capital of Berlin, said he did not know who was responsible and there was no evidence Saudi nationals were involved.
READ MORE: Gunmen attack Iran's parliament, Khomeini shrine
Ghanbar Naderi, journalist of Kayhan International, speaking from Tehran, told Al Jazeera that "people don't believe that ISIS is acting alone".
"They have patrons, they have associates in the Middle East and beyond. Saudi Arabia is one of the main suspects because of its ideology that goes well with what the ISIS fighters feel, think and believe," he said.
