They're a combination of the UN (the whole, not just UNC) and NATO.

@Nomad416 Please take a look at the logo of their organization,because the United Nations Command in the real world also uses the flag to resemble the equivalent of Allied Nations.In Mercenaries: Playground of Destruction,also, their logo has the same sixteen stars.Please refer to their military organization webpage link https://www.unc.mil/Resources/Fact-Sheets/Article/2468599/united-nations-command-logo-information/