The expansion, subtitled Call of Pripyat, is due out later this year, according to ClanBase and TTLG. Both sites, originally noticed by Blue's News, cite the same Russian language interview with GSC Game World president Sergey Grigorovitch.
S.T.A.L.K.E.R. 2 was also mentioned as being in the works, but no details were offered.
The post-apocalyptic first-person role-playing series premiered in 2007 with S.T.A.K.E.R.: Shadow of Chernobyl in 2007, followed in 2008 by prequel Clear Sky. While the franchise has only hit PC thus far, GSC is a licensed Xbox 360 developer.