"James is a professional journalist who has remained totally neutral in this conflict," Hoog said. "His captors, whoever they may be, must release him immediately."
In April 2011, Foley and two other reporters were detained by government forces in Libya while covering that country's civil war. They were released six weeks later. South African photographer Anton Hammerl was shot during their capture and left to die in the desert.
"I'll regret that day for the rest of my life. I'll regret what happened to Anton," Foley told The Associated Press at the time. "I will constantly analyze that."
The U.N. said Wednesday that more than 60,000 people have been killed since the start of Syria's conflict in March 2011. This number represents a large jump from death tolls previously given by anti-regime activists.
The Committee to Protect Journalists said that Syria was the most dangerous country in the world for journalists in 2012, when 28 reporters were killed.
Those who lost their lives include award-winning French TV reporter Gilles Jacquier, photographer Remi Ochlik and Britain's Sunday Times correspondent Marie Colvin. Also, Anthony Shadid, a correspondent for The New York Times, died after an apparent asthma attack while on assignment in Syria.
Last month, NBC correspondent Richard Engel and his crew were detained by pro-regime gunmen near where Foley was kidnapped. After his release, Engels said they escaped unharmed during a firefight between their captors and anti-regime rebels.
Associated Press writer Elaine Ganley contributed reporting from Paris.