Income generated by LLC with US operation may be considered as 'effectively connected with US trade or business' and as such be subject to US taxation, even if all LLC members are non US residents and living outside of the US.
The income will be subject to Federal tax and to the state tax in which the US operation is located. For example if you have a Delaware LLC, with office and employee in NY and another office and employee California, you will be subject to Federal tax on income from both properties, New York tax on the portion of the taxable income associated with New york (normally it will be calculated as follows : New York wages / Total wages x Total taxable income), and California tax on the portion of the taxable income associated with California (normally it will be calculated as follows : California wages / Total wages x Total taxable income).
