Iran threatened the European Union with consequences on Sunday if parliament adds the Revolutionary Guards (IRGC) to a terror list, as proposed.
"The European Parliament has shot itself in the foot and the response will be a countermeasure," Iran's Foreign Minister Hussein Amirabdollahian tweeted.
The Iranian chief diplomat confirmed that the parliament was already working on the draft.
Amirabdollahian and IRGC commander Hussein Salami attended a meeting on Sunday to discuss the issue.
The exact countermeasures are unclear.
There was talk however, of detaining foreign oil tankers in the Persian Gulf or even the Strait of Hormuz, which lies between the Persian Gulf and the Gulf of Oman. It is one of the most important shipping routes worldwide, especially for oil.
The IRGC are Iran's elite armed forces. Established after the 1979 Islamic Revolution, the unit is supposed to prevent a coup and protect state ideology.
It has come under increasing criticism for its involvement in suppressing recent unrest in Iran, demonstrating the government's strict rule.
Many Iranians and politicians in Europe are now calling for the Revolutionary Guards to be labelled a terrorist organization.
The EU has already imposed sanctions on many high-ranking officers of the Revolutionary Guards. They plan to adopt more at a meeting on Monday, diplomats said.
Iran's Foreign Ministry already strongly condemned the EU's decision on Thursday, calling the MEPs "political dwarfs" who would bitterly regret their decision.
The state-run daily on Sunday even suggested severing diplomatic and economic relations with all European states in favour of the measure.
