  We study numerically low-order harmonic generation in noble gases pumped by
intense femtosecond laser pulses in the tunneling ionization regime. We analyze
the influence of the phase-mismatching on this process, caused by the generated
plasma, and study in dependence on the pump intensity the origin of harmonic
generation arising either from the bound-electron nonlinearity or the
tunnel-ionization current. It is shown that in argon the optimum pump intensity
of about 100 TW/cm$^2$ leads to the maximum efficiency, where the main
contribution to low-order harmonics originates from the bound-electron third
and fifth order susceptibilities, while for intensities higher than 300
TW/cm$^2$ the tunnel-ionization current plays the dominant role. Besides, we
predict that VUV pulses at 133 nm can be generated with relatively high
efficiency of about $1.5\times10^{-3}$ by 400 nm pump pulses.
