An EU plan to buy oil from militant-held areas of Syria is illegal and an "act of aggression," the Syrian foreign ministry warned on Tuesday in letters to the United Nations.

An EU plan to buy oil from militant-held areas of Syria is illegal and an "act of aggression," the Syrian foreign ministry warned on Tuesday in letters to the United Nations.

"In an unprecedented decision that contradicts international law and the UN Charter... the European Union has decided to allow member states to import petrol... under the pretext of supporting the opposition," state news agency SANA reported, citing the letters.

"It is an illegal decision and an act of aggression."

Militant opposition groups fighting the President Bashar al-Assad's troops won a fresh boost Monday when the European Union eased its oil embargo to let them exploit the resources they control.

But the decision raised a furious response in Damascus.

The European Union will be trading "with the so-called opposition Coalition, which represents no one in Syria," the letters to UN Secretary General Ban Ki-moon and the Security Council said.

The decision is an act of "complicity in the theft of resources that belong to the Syrian people, represented by the current, legitimate government," they added.

"The European Union is following its political and economic campaign that targets the national economy and the daily bread of Syrian citizens," the ministry added, referring to EU sanctions on the Assad regime.

EU ministers' decision to ease the 2011 oil embargo will enable companies on a case-by-case basis not only to import Syrian crude but also to export oil production technology and investment cash to areas in the hands of the militant opposition groups.

Under the deal, European firms seeking to import Syrian crude or invest in the energy sector would ask for authorization from their government, which in turn would confer with the so-called National Coalition to secure its agreement.

At the start of the unrest that broke out in March 2011, Syria's oil production was slashed by almost two thirds, falling to 130,000 barrels a day in March, just 0.1 percent of the world's total production, according to the International Energy Agency's latest estimates.