  We investigate the production of an isolated attosecond pulse~(IAP) via the
phase-matching gating of high-harmonic generation by intense laser pulses. Our
study is based on the integration of the propagation equation for the
fundamental and generated fields with nonlinear polarisation found via the
numerical solution of the time-dependent Schr\"odinger equation. We study the
XUV energy as a function of the propagation distance (or the medium density)
and find that the onset of the IAP production corresponds to the change from
linear to quadratic dependence of this energy on the propagation distance (or
density). Finally, we show that the upper limit of the fundamental pulse
duration for which the IAP generation is feasible is defined by the temporal
spreading of the fundamental pulse during the propagation. This nonlinear
spreading is defined by the difference of the group velocities for the neutral
and photoionised medium.
