Syrian and Russian air strikes on Aleppo have prompted accusations of war crimes.
It is not clear exactly what armaments have been deployed, but UN Secretary-General Ban Ki Moon cited reports of incendiary weapons and bunker buster bombs, while barrel bombs and cluster munitions have also been used previously in Syria.
Which weapons might have been used?
Bunker busters: named for their use in penetrating hardened targets such as underground military headquarters. Incendiary weapons: used to start fires, including materials such as napalm and white phosphorous, which can cause severe burns if they come into contact with skin. They have a legitimate function of generating smoke-screens.
Thermobaric bombs, also known as fuel-air explosives, set a fire that sucks the oxygen out of underground spaces and burns everything in its path. Cluster munitions are internationally-banned and release "bomblets" over a wider area.
Barrel bombs are unguided, improvised weapons with a wide impact, often made from an oil drum filled with explosives and metal fragments and dropped from the air.
Why would they be controversial in Aleppo?
"The use of weapons in armed conflict falls under international humanitarian law, which prohibits the direct targeting of civilians and prohibits indiscriminate and disproportionate attacks," said Hannah Brice, assistant head of International Security at the Chatham House think-tank. "The issue with many of the explosive weapons being used in Syria and in Aleppo is their use in residential areas where there is a high civilian population." She added: "The issue however is complicated when military targets are located within civilian areas."
Brice quoted a report by the NGO Action on Armed Violence which found that when explosive weapons are used in populated areas, 92 per cent of those killed and injured are civilians. They also often destroy critical infrastructure, such as healthcare facilities, sanitation, water and power supplies.
Ben Goodlad, principal weapons analyst at IHS Jane's, said: "Both bunker buster and fuel-air explosive munitions are intended for use against hardened, difficult to strike targets. The high level of explosive and blast effects caused by these munitions would have a devastating effect on a built-up area with collateral damage being almost unavoidable."
Stephen Rapp, former US ambassador for war crimes, said "any incendiary devices are not going to be in a situation where they can distinguish between (military targets and civilians). They are going to cause horrible loss of life to civilians".