Reporter Nicolas Henin, 37, and photographer Pierre Torres, 29, were captured on June 22 while working in the northern city of Raqqa, according to their families.
None of the armed groups who were fighting for control of the town at the time have claimed the two men as hostages or made any demands related to them, the families said, adding that French authorities had informed them the two men were still alive in August.
The abduction was not initially made public in the hope that a media blackout might help facilitate a quick release.
"But after more than 100 days of waiting, the families and friends of the two journalists wanted to address a message to Nicolas and Pierre to tell them they are doing everything they can to secure their rapid release," a statement from the families said.
They said they would now be working with the families of the two other detained French journalists, Didier Francois and Edouard Elias.
Francois, a seasoned war reporter for Europe 1 radio, and Elias, a photographer, were detained on June 6 by unknown men at a checkpoint while travelling to Syria's second city of Aleppo.
French government sources said there were now 11 nationals held hostage worldwide.
President Farncois Hollande reaffirmed Wednesday that Paris was doing all it could to secure their release, government spokeswoman Najat Vallud-Belkacem said.
Prime Minister Jean-Marc Ayrault said Paris had recent indications that Francois and Elias were still alive.
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Ayrault also made reference during an interview on Europe 1 radio to the detention of Henin and Torres, inadvertently messing up the plans of the families.
They had not intended to go public with the news until Saturday, with an announcement scheduled to take place at the Prix Bayeux awards, where Henin is nominated for a prize for his work.
"I was astonished that Ayrault released the news before the moment envisaged by the families," said Eric de Lavarene, an executive at Solas Films, the TV production firm which employed Henin. "It was a bit of a gaffe, it shows a real lack of tact."
Through Solas, Henin regularly did work for French television station Arte, weekly magazine Le Point, Belgium and Switzerland's national broadcasters and Radio-Canada.
Henin has made five trips to Syria since 2011.
Torres had covered the 2011 Libyan conflict and was on his second trip to Syria. Among his clients were AFP.
International press freedom watchdog Reporters Without Borders (RSF) describes Syria as currently the world's most dangerous country for journalists to work in. Since the conflict began in March 2011, RSF has recorded the deaths of 25 journalists and 26 citizen journalists.
The fate of the hostages may depend on who is holding them.
France has considerable influence with the official leadership of the Syrian opposition, having championed their cause on the international stage.
But Paris has no influence over the Islamist groups who have become increasingly influential within the rebel coalition in recent months or with the regime.
Paris angered President Bashar al-Assad by publicly backing calls for air strikes against the regime in response to a chemical weapons attack in August that has been widely blamed on Damascus.
Although Syria's agreement to hand over its chemical weapons arsenal has put air strikes on hold, France continues to pursue a strong anti-Assad line, insisting that the Syrian strongman should be hauled before the International Criminal Court to answer for his conduct during the conflict.