Intel Fortran and OpenMP Error for Mex on Windows

I am trying to add an openmp speed up to a loop in a Fortran mex file. On compiling, I get the following error:

 error LNK2019: unresolved external symbol omp_get_num_threads referenced in function MEXFUNCTION

My compiler command is: mex FFLAGS="\$FFLAGS -ffree-line-length-none /Qopenmp -heap-arrays -O3" LDFLAGS="\$LDFLAGS /Qopenmp -heap-arrays /iface:cvf" file_name.f90

In the subroutine, I use omp_lib and call omp_get_num_threads as follows: !$OMP PARALLEL SHARED(var_names), PRIVATE(pp), NUM_THREADS(4)

pp = OMP_GET_NUM_THREADS()

My Matlab version is R2012a with Intel Visual Fortran 12 (with Microsoft Software Development Kit (SDK) linker)

Does anyone know what the issue is?

Thanks