  We discuss the contribution of ultraviolet (UV) renormalons in QCD to
two-point functions of quark current operators. This explicitly includes
effects due to the exchange of one renormalon chain as well as two chains. It
is shown that, when the external euclidean momentum of the two-point functions
becomes smaller than the scale $\Lambda_L$ associated with the Landau
singularity of the QCD one-loop running coupling constant, the positions of the
UV renormalons in the Borel plane become true singularities in the integration
range of the Borel transform. This introduces ambiguities in the evaluation of
the corresponding two-point functions. The ambiguities associated with the
leading UV renormalon singularity are of the same type as the contribution due
to the inclusion of dimension d=6 local operators in a low-energy effective
Lagrangian valid at scales smaller than $\Lambda_L$. We then discuss the
inclusion of an infinite number of renormalon chains and argue that the
previous ambiguity hints at a plausible approximation scheme for low-energy
QCD, resulting in an effective Lagrangian similar to the one of the extended
Nambu-Jona-Lasinio (ENJL) model of QCD at large N_c.
