Chemical Warfare
Drones as mentioned by @Separatrix are an excellent way to minimize your casualties, but are ineffective when people create tunnels, adapt to living in caves, and start a guerrilla style warfare if and when you choose send boot on the ground. Your first and foremost job would be to surround the city, to minimize smuggled resources, forcing people to rely entirely upon only available resources.
Disrupt or destroy electric grids (Artillery/Bombs), water supply (through poisoning), hospitals and any government facilities which would contain gas masks, or medicines. Following a co-ordinated heavy artillery and drone shelling in the midst of ensuing chaos, use either aircraft bombs or binary munitions to completely cover the entire city with Nerve Gas or Asphyxiate. You also have a wide variety of chemicals to choose from. Most are odorless, colorless, and practically undetected till effects start. Most notable ones are Sarin, Novichok, and VX (V Series).
This not only ensures high number of causalities, crushes their structure but also, the sight of mutilated corpse with blisters, and agony of the affected but alive population does irreparable damage to their moral, allowing for a much easier surrender. If you are still unsure about it, even Hitler choose not to use mustard/Chlorine gas in WW2 after seeing its horrifying effects in WW1.
