In reply to Mandeep:
I assume you are using bind to instantiate the interface.
You cannot connect a variable to an inout port. Only nets allowed on inouts because bi-directional ports imply multiple drivers, which only nets can have.
You can connect variables to a port passed by reference, similar to a task/function argument using the [=courier]ref[/port] direction. Note that passing by reference requires stricter identical datatype matching
interface A_if(ref reg [1:0] state_reg);
force state_reg = 2'b11;
You can also use a trick involving upward name referencing. Since you know the name of the module you are binding the interface into, you can eliminate the interface port altogether and do this
force A.state_reg = 2'b11;