Analysis of satellite imagery of a deadly attack on an aid convoy in Syria last month showed that it was an air strike, a UN expert said on Wednesday.

At least 20 people were killed in the attack on the UN and Syrian Arab Red Crescent convoy at Urem Al-Kubra near the northern city of Aleppo which destroyed 18 of 31 trucks, a warehouse and clinic.

The United States blamed two Russian fighter jets that it said were in the skies above the area at the time of the incident. Moscow denies the charge and says the convoy caught fire.

"With our analysis we determined it was an air strike and I think multiple other sources have said that as well," Lars Bromley, research adviser at UNOSAT, told a news conference.

Einar Bjorgo from United Nations Institute for Training and Research shows a satellite image of the attack [EPA]

"For air strikes, what you are usually looking out for is the size of the crater that is visible and the type of crater," he said. A giant crater on the ground was caused "almost certainly [by] air dropped munitions" as opposed to artillery or mortars, he said.

The UN has so far referred only to an "attack", which led to a brief suspension of its land convoys in Syria, while the International Federation of Red Cross and Red Crescent Societies initially referred to "air strikes" in a statement.

Battle for Aleppo 

UN Secretary-General Ban Ki-moon announced on Friday that he would establish an internal UN board of inquiry to investigate the attack and urged all parties to fully cooperate.

UNOSAT, which reviews only commercially-available satellite images, has not been asked to share its analysis with the UN inquiry, but is prepared to do so, UNOSAT manager Einar Bjorgo said. "Our images are from time to time used in order to brief Security Council members," he told the briefing.

The UN's claim about the Aid convoy attack in Aleppo came as the battle for the key province intensified.

Three children were among at least 19 civilians killed in an air strike on a village held by the Islamic State or Iraq and the Levant (ISIL, also known as ISIS) group in northern Aleppo province on Wednesday, a monitor said.

The Syrian Observatory for Human Rights said it was unclear whether the strike was carried out by the US-led coalition fighting ISIL, or Turkey, which is leading an operation against the group in the area with support from Syrian rebel forces.

The strike hit the village of Thalthana, in northern Aleppo province, the Britain-based monitor said.

Source: Agencies