No module named _cffi_backend

I have Python 2.6 in my Linux rhel-5. I have installed pip and required CFFI packages. When I try to run a sample CFFI program:

ffi = FFI()

it says:

File "/usr/lib/python2.6/site-packages/cffi/api.py", line 56, in __init__    import _cffi_backend as backendImportError: No module named _cffi_backend

What could be the possible error? Did I miss something during installation? I have installed pip, wheel, pycparser, pytest and cffi.