I want to use an iPhone5 S bought in the US in India I bought an iPhone 5S from the US. It is unlocked. In India I am unable to use it with the local (Indian) service provider. When I insert a nano sim card, I am getting an error msg: "activation required".

Please explain how I could use it with local GSM service provider.
If I remember correctly, the "unlocked" iPhone 5S sold in the US is really unlocked for use on TMO or AT&amp;T.  This means you need to activate it with one of those two carriers before it can be used with another carrier's SIM card.  I'd contact Apple to verify and see if they can do anything to help out since you're no longer in the US.