Dr Akram Gliwan, spokesman for a hospital in Misrata where pro-government fighters are treated, told AFP that photographer Jeroen Oerlemans was "shot in the chest by an IS sniper while covering battles in Sirte" 450 kilometres east of Tripoli.
Gliwan said his body had been transferred to Misrata, 200 kilometres west of Sirte.
Oerlemans was working in Libya for a number of organisations, including the Belgian weekly Knack magazine, which confirmed his death.
A message on Knack's website said Oerlemans was shot on a reporting assignment and that the publication "wishes his family much strength".
IS fighters holed up in Sirte, the birthplace of ousted Libyan dictator Moamer Kadhafi, responded with suicide bombings and sniper fire, slowing the government-backed advance.
Fighting today killed at least 10 IS fighters and eight pro-government fighters around Sirte, the unity government in Tripoli said.