Iran will take legal action if Washington tries to stop newly acquired Iranian passenger planes flying international routes, the head of the country's aviation authority said on Tuesday.

Iran's Mahan Air, blacklisted by Washington, bought eight second-hand Airbus A340s and one Airbus A321 in May in defiance of U.S. sanctions. Iran's semi-official Fars news agency reported last week that the planes would be used on international routes.

The U.S. Treasury department said last month that Mahan Air had a "blockable interest" in the planes, meaning they could be subject to an asset freeze, raising the possibility that Washington may attempt to have them seized at airports outside Iran.

"As long as ... a disruption has not taken place, we cannot make a definite assertion. But if it happens, Iran will take legal action," said Alireza Jahangirian, head of Iran's Civil Aviation Organization, according to the Iranian Students' News Agency (ISNA).

"America does not have the right to stop our planes on international trips ... Under international laws, it is impossible to seize the new Iranian aircraft."

The U.S. Treasury has imposed sanctions on two firms based in Iraq and the United Arab Emirates on suspicion of helping the purchase.

Jahangirian said the planes would join Iran's aviation fleet "in the coming weeks", after receiving their documentation.

Fars reported on June 1 that the four-engined A340s would start flying from Tehran to Dubai and Istanbul, and would then be used on long-haul routes.

Aviation experts say Iran is interested in aircraft such as the A340 because it lacks the approvals needed to fly two-engined aircraft on long-haul routes following years of relative isolation in the global aviation market.

Iran has been hit by sanctions from the United States, the European Union and the United Nations over its disputed nuclear program. The U.S. sanctions ban the sale of aircraft and parts to the Islamic Republic.

Under an interim deal reached between Iran and six major powers in 2013, the ban on the sale of spare parts was eased, but the sale of aircraft remains restricted.

However, Iran and the powers are in talks to clinch a final deal by June 30 to end the decade-old standoff and usher in an end to sanctions.

Iran's airlines have been plagued by crashes, which Iranian politicians blame on the sanctions.

Iran's 189 active passenger aircraft have an average age of 22 years, according to the British aviation consultancy Ascend. It also has 76 in storage with an average age of 24 years. They serve a country of 76 million people.

By Reuters