  (Abridged) Context. Massive star formation is associated with energetic
processes, which result in significant gas cooling via far-infrared (IR) lines.
Velocity-resolved observations can constrain the kinematics of the gas,
allowing the identification of the physical mechanisms responsible for gas
heating. Aims. Our aim is to quantify far-infrared CO line emission toward
high-mass star-forming regions, identify the high-velocity gas component
associated with outflows, and estimate the physical conditions required for the
excitation of the observed lines. Methods. Velocity-resolved SOFIA/GREAT
spectra of 13 high-mass star forming clumps of various luminosities and
evolutionary stages are studied using CO 11-10 and 16-15 lines. Results. All
targets show strong high-J CO emission in the far-IR, characterized by broad
line wings associated with outflows, thereby significantly increasing the
sample of sources with velocity-resolved high-J CO spectra. The contribution of
the emission in the line wings does not correlate with the envelope mass or
evolutionary stage. Gas rotational temperatures cover a narrow range of 120-220
K for the line wings. The non-LTE radiative transfer models indicate gas
densities of 1e5-1e7 cm-3 and N(CO) of 1e17- 1e18 cm-2, similar to physical
conditions in deeply-embedded low- and high-mass protostars. The
velocity-integrated CO line fluxes correlate with the bolometric luminosity
over 7 orders of magnitude including data on the low-mass protostars,
suggesting similar processes are responsible for the high-J CO excitation over
a significant range of physical scales. Conclusions. Velocity-resolved line
profiles allow the detection of outflows toward massive star-forming clumps
spanning a broad range of evolutionary stages. The lack of clear evolutionary
trends suggest that mass accretion and ejection prevail during the entire
lifetime of star-forming clumps.
