In Syria, where over the past two days Russian jets have flown over Iraq and into Syrian areas, conducting a series of bombing raids around Aleppo. The Russian jets are stationed out of an Iranian air base. The US State Department's deputy spokesman on Wednesday said recent Russian air strikes were unhelpful and even dangerous.

"Fundamentally, this isn't helpful - and it's not helpful because, you know, it continues to complicate an already dangerous situation in and around Aleppo, when you have Russia using Iranian air bases as a way to carry out more intensive bombing runs that are hitting - continue to hit - civilian populations. And so, our concerns remain very vivid," , US State Department deputy spokesman Mark Toner said.

Iran on Tuesday allowed Russian warplanes to take off from its territory to bomb targets in Syria - the first such occasion since Russia launched its operation in the war-torn Arab nation last year. On Wednesday, Russia conducted another bombing raid against targets in Syria using an Iranian air base.

The US also suggested that Russia's co-operation with Tehran might violate a UN resolution, which prohibits the supply, sale and transfer of combat aircraft to Iran.