Context : Young massive stars and stellar clusters continuously form in the Galactic disk , generating new H ii regions within their natal giant molecular clouds and subsequently enriching the interstellar medium via their winds and supernovae .

Aims : Massive stars are among the brightest infrared stars in such regions ; their identification permits the characterisation of the star formation history of the associated cloud as well as constraining the location of stellar aggregates and hence their occurrence as a function of global environment .

Methods : We present a stellar spectroscopic survey in the direction of the giant molecular cloud G23.3 - 0.3 .
This complex is located at a distance of \sim 4 - 5 kpc , and consists of several H ii regions and supernova remnants .

Results : We discovered 11 Of _ { K } ^ { + } stars , one candidate Luminous Blue Variable , several OB stars , and candidate red supergiants .
Stars with K -band extinction from \sim 1.3 - 1.9 mag appear to be associated with the GMC G23.3 - 0.3 ; O and B-type s satisfying this criterion have spectrophotometric distances consistent with that of the giant molecular cloud .
Combining near-IR spectroscopic and photometric data allowed us to characterize the multiple sites of star formation within it .
The O-type stars have masses from \sim 25 - 45 M _ { \odot } , and ages of 5-8 Myr .
Two new red supergiants were detected with interstellar extinction typical of the cloud ; along with the two RSGs within the cluster GLIMPSE9 , they trace an older burst with an age of 20–30 Myr .
Massive stars were also detected in the core of three supernova remnants - W41 , G22.7 - 0.2 , and G22.7583 - 0.4917 .

Conclusions : A large population of massive stars appears associated with the GMC G23.3 - 0.3 , with the properties inferred for them indicative of an extended history of stars formation .