Military blogs are starting to compare the siege at Mosul with earlier battles. We have to include the World War II fight at Stalingrad in this conversation. That was likely the biggest and the baddest clash of mechanized armies of all time. In terms of casualties the military losses will not match up because the forces at Mosul are smaller by factors of 20 and 100 than the titanic forces that battered each other at Stalingrad. For civilian losses, on the other hand, Mosul has a likely 750,000 people still in the city boundaries with DAESH killing those who try to escape.
Aleppo is different altogether. The battle there is now a simple siege. Civilians die by the hundreds a month, not tens of thousands, and there are now eight open gates to get out. Families are fleeing Aleppo and the al-Qaeda/Nusrah and Brotherhood defenders are inept at stopping it.
By way of comparison the cities of Stalingrad (now Volgograd, formerly Tsaritzin) and Mosul are roughly comparable in pre-war size, population, and economic productivity. Lots of stuff was being manufactured at Mosul. This was a modern city
Stalingrad and Mosul. Sensible estimates:
850,000 population in 1940. Down to 1,500 after the battle. // As many as 1,500,000 in 2014. Now down to 750,000 or possibly as low as 400,000.
1,600,000 Axis troops vs. 1,200,000 Russians // 75,000 Allied troops vs. 6,000 DAESH fighters and suicide bombers
10,250 Axis artillery pieces vs. 13,450 Russian artillery pieces // 600 Allied artillery pieces vs. ???? DAESH
500 Axis tanks vs. 900 Russian tanks // <100 Allied tanks vs. nothing. [DAESH never figured out using its captured tanks.]
300 Axis aircraft vs. 600 Russian aircraft // 75 Allied aircraft vs. nothing.
The Battle of Stalingrad lasted from August 1942 to February 1943 // Mosul is expected to fall in 2 months to 4 months max.
Germany's Sixth Army attacked Stalingrad. Eventually the Russian counterattack succeeded in surrounding the whole of that army. Surround-and-annihilate was applied with few prisoners taken on a day to day basis.
Eventually 91,000 Sixth Army soldiers and officers were captured at the final collapse. Between 5,000 and 6,000 of these men survived, 5,000 returned home in the 1950s.
DAESH at Mosul can expect similar extermination. For civilians, unfortunately, survival could prove impossible if the battle goes on beyond a couple of months. Water and food will run out. "Humanitarian pause" is very unlikely where DAESH is a player.
In Stalingrad the population started evacuation before Sixth Army brought in heavy artillery -- Mosul's population is forcibly prevented from taking off. In Mosul the Allies will have to capture whole neighborhoods to enable local evacuations. Gaming the Mosul battle, civilian losses can rise into the 200,000 to 300,000 range. It's that bad.