  Aims. We present MCFOST-art, a new non-local thermodynamic equilibrium
radiative transfer solver for multilevel atomic systems. The code is embedded
in the 3D radiative transfer code MCFOST and is compatible with most of the
MCFOST modules. The code is versatile and designed to model the close
environment of stars in 3D. Methods. The code solves for the statistical
equilibrium and radiative transfer equations using the Multilevel Accelerated
Lambda Iteration (MALI) method. We tested MCFOST-art on spherically symmetric
models of stellar photospheres as well as on a standard model of the solar
atmosphere. We computed atomic level populations and outgoing fluxes and
compared these values with the results of the TURBOspectrum and RH codes.
Calculations including expansion and rotation of the atmosphere were also
performed. We tested both the pure local thermodynamic equilibrium and the
out-of-equilibrium problems. Results. In all cases, the results from all codes
agree within a few percent at all wavelengths and reach the sub-percent level
between RH and MCFOST-art. We still note a few marginal discrepancies between
MCFOST-art and TURBOspectrum as a result of different treatments of background
opacities at some critical wavelength ranges.
