Russia has said its use of an Iranian airbase to carry out bombing missions in Syria does not violate a UN resolution that forbids supplying fighter jets to Tehran.

"There has been no supply, sale or transfer of combat jets to Iran," Russian Foreign Minister Sergei Lavrov said on Wednesday in comments reported by the Russian state news agency TASS.

"These jets are being used by Russian Air forces with Iranian consent within an anti-terrorist operation in Syria on the request of the legal Syrian government."

Lavrov was responding to comments by US State Department spokesman Mark Toner, who said on Tuesday that Russia might be violating UN Security Council Resolution 2231 by using Iranian territory to launch air raids in Syria.

The resolution bans the supply, sale or transfer of combat aircraft to Iran unless approved in advance by the UN Security Council.

Toner also described Russia's move as "unfortunate but not surprising or unexpected", saying the air strikes that Russia says target the ISIL in Syria actually "predominately target moderate Syrian opposition forces".

Al Jazeera's Reza Sayah, reporting from Gazientep on the Turkey-Syria border, said recent opposition victories, including the breaking of the siege of opposition-held Aleppo, may have been behind Russia's decision to use the air base.

"In recent weeks, the opposition forces have gained some momentum, including the breaking of the regime's siege of Aleppo. The recent battlefield victories are another reason why some expect the Russian air campaign to intensify with the help from the air base in Iran."

On Tuesday, Russian fighter jets used an Iranian airbase for the first time for military actions in Syria, TASS reported.

Abdul Jabbar al-Okaidi, an opposition leader and spokesman for the Free Syrian Army in Aleppo told Al Jazeera that the Russian move was "not a surprise."

"Russia and Iran are partners in the killing of the Syrian people and partners in support of Assad. There is no reason to be surprised. This has become a game. First it was the Americans, then the Russians and now the Iranians. The only losers are the Syrian people who are being killed in the name of fighting terrorism."

Russia and Iran are longtime allies of Bashar al-Assad.

PHOTO CAPTION

A still image, taken from video footage and released by Russia's Defense Ministry on August 16, 2016, shows a Russian Tupolev Tu-22M3 long-range bomber based in Iran, flying after bombs were dropped off, at an unknown location in Syria (Reuters)

Al-Jazeera