Almost half of those who died were residents without ties to the gangs which are fighting for control of the Cité Soleil neighbourhood, the UN says.
Locals say they are running out of drinking water and food as deliveries have been halted amid the shoot-outs.
One resident described his life as "a cycle of fear, stress and despair".
Gang violence had already shot up since the assassination of President Jovenel Moïse by mercenaries a year ago, but it has reached shocking new levels since a battle erupted on 8 July between two criminal alliances, known as G9 and G-Pèp.
The UN says that 209 people were killed between 8 and 17 July, of which 114 were gang members. A further 254 people have sustained gunshot wounds, more than half of them residents without links to the gangs.