Jose Manuel Lopez, Antonio Pampliega and Angel Sastre entered Syria on July 10 "and there has been no news of them since July 12", Elsa Gonzalez, president of the Federation of Press Associations of Spain, told national television.
"In that region there is intense fighting going on, so there is cause for concern," she said, but added: "For the moment we can only call it a disappearance."
Pampliega, a freelance journalist covering war zones, contributed to AFP's text coverage of Syria for a period up to 2013.
According to data on the website of the Madrid Press Association, Sastre has worked in various trouble spots around the world for Spanish television, radio and press.
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El Mundo correspondent Javier Espinosa and freelance photographer Ricardo Garcia Vilanova were held by the Islamic State group for six months and freed in March 2014.