Hierarchical reference not allowed from within a package.

Hi All,
We are trying this kind of example. We are having two files of these. It's giving  "Hierarchical reference  not allowed from within a package." at    class1_instance.class1_function(.....)     class2_instance.class2_function(......)

I tried passing both classes as members of a structure. but it giving same error when accessing member class functions from structure.

What's the solution to access functions of these classes in Program main Blocks.

file1.sv
_______________________

program main;

class1 class1_instance;
class 2 class2_instance;
initial begin
    class1_instance=new;
    class2_instance=new;
    call_task_from_other_file(class1_instance, class2_instance);
end
endprogram


file2.sv
________________________
task call_task_from_other_file(class1 class1_instance, class2 class2_instance);
    class1_instance.class1_function(.....)
    class2_instance.class2_function(......)
endtask




Regards
Mayank