Recent high resolution numerical simulations have suggested that the interstellar atomic hydrogen clouds have a complex two-phase structure .
Since molecular clouds form through the contraction of HI gas , the question arises as to whether this structure is maintained in the molecular phase or not .
Here we investigate whether the warm neutral atomic hydrogen ( WNM ) can exist in molecular clouds .
We calculate how far a piece of WNM which is not heated by the UV photons could penetrate into the cloud , and find that in the absence of any heating it is unlikely that large fraction of WNM survives inside high pressure molecular clouds .
We then consider two possible heating mechanisms , namely dissipation of turbulent energy and dissipation of MHD waves propagating in the WNM inside the cloud .
We find that the second one is sufficient to allow the existence of WNM inside a molecular cloud of size \simeq 1 pc having pressure equal to \simeq 10 \times P _ { ISM } .
This result suggests the possibility that channels of magnetised WMN may provide efficient energy injection for sustaining internal turbulence which otherwise decays in a crossing time .