dotnet publish "Unable to load the service index for source https://api.nuget.org/v3/index.json"

I am getting the error message

Unable to load the service index for source https://api.nuget.org/v3/index.json'

on executing dotnet publish. dotnet restore is working fine with leveraging a local NuGet repository (Artifactory). The system I am running this on has no internet connection. But why does dotnet publish reach out to nuget.org? And is there a way to prevent this happening?

D:\tools\dotnet\dotnet.exe publish proj.sln --configuration Release --output bin/Release/PublishOutput
D:\tools\dotnet\sdk\2.0.0\NuGet.targets(102,5): error : Unable to load the service index for source https://api.nuget.org/v3/index.json[D:\buildAgent_adetca06\work\5a1e5e99f083b612\proj.sln]
D:\tools\dotnet\sdk\2.0.0\NuGet.targets(102,5): error : An error occurred while sending the request. [D:\buildAgent_adetca06\work\5a1e5e99f083b612\proj.sln]
D:\tools\dotnet\sdk\2.0.0\NuGet.targets(102,5): error : The operation timed out [D:\buildAgent_adetca06\work\5a1e5e99f083b612\proj.sln]

My .NET enviroment

dotnet --info
.NET Command Line Tools (2.0.0)

Product Information:
 Version:            2.0.0
 Commit SHA-1 hash:  cdcd1928c9

Runtime Environment:
 OS Name:     Windows
 OS Version:  6.3.9600
 OS Platform: Windows
 RID:         win81-x64
 Base Path:   D:\tools\dotnet\sdk\2.0.0\

Microsoft .NET Core Shared Framework Host

  Version  : 2.0.0
  Build    : e8b8861ac7faf042c87a5c2f9f2d04c98b69f28d