Image copyright Reuters

Western forces are using satellites and unmanned aircraft to gather evidence of possible war crimes in Syria, the UK government has confirmed.

Thousands of civilians remain trapped in Aleppo as fighting continues.

The Foreign Office said no UK government drones were operating over Aleppo, but the Syrian opposition had been trained to collect evidence.

Foreign Office minister Tobias Ellwood said those committing crimes against humanity may "be held to account"

Former chancellor George Osborne told MPs in an emergency Commons debate on Tuesday that the situation in Aleppo was due to "a vacuum of Western and British leadership".

Also on Tuesday, the UN said civilians were being killed deliberately by militias supporting President Bashar al-Assad's government.

The government has said that as well as using aerial surveillance, evidence of alleged atrocities is being collected from open source social media and local testimony from activists on the ground.

In a statement, the Foreign Office Minister for the Middle East, Tobias Ellwood, said the UN security council had "failed" Aleppo by denying UN humanitarian access to the city.

"Russia has gone from supporting siege to supporting slaughter. Yet Russia's veto will not prevent individuals committing potential crimes against humanity from being held to account," he added.

"Members of Assad's regime participating at any level in these atrocities should be aware data is now being collected through testimonials, social media and aerial technology - so individuals committing crimes against humanity may one day be tapped on the shoulder and held account for their actions."

Image copyright PA Image caption Foreign Secretary Boris Johnson said on Tuesday that President Assad "was allowed to do his worst"

Government sources earlier said the lesson learned from Iraq was to gather information about alleged war crimes as a conflict takes place.

This gives investigators something to work with once the fighting stops, so that eventually those guilty can be brought to justice in the international criminal courts.

The Ministry of Defence has said it does not comment on intelligence gathering operations.