  W51A is the most active star formation region of the Giant \ion{H}{2} region
W51. It harbors the two massive proto-clusters W51e and W51\,IRS2, which are
very rare in the Galaxy. We aim to identify the new born massive stars and
UCHII regions to derive its distance and age. We performed IFU observations
with NIFS+ALTAIR of nine targets in the W51A sub-region. The distance modulus
was obtained using the spectral classification in the $K$-band and a reddening
law appropriate to the inner Galactic plane. We derived the distance and the
spectral types for five of them, ranging from O8 to O9.5, similar to those
derived from radio continuum data, except for two sources that we assigned
somewhat a later spectral type. We included another seven objects with precise
spectral classification from other works, which allowed us to better constrain
the distance estimate. Our spectrophotometric distance d=4.80\,$\pm$\,1.27\,kpc
is in good agreement with those derived from the Galactic rotation model and
trigonometric parallaxes, placing the region near the tangent point of the
Sagittarius arm. We conclude that the stars studied in this work have an age
spread of 1.5-4 Myr, substantially older than thought to date.
