The international players union FIFPro has written to Blatter saying that it remains "deeply concerned about Belounis' precarious situation" and calling for action.
It said Belounis, 33, is stranded in Qatar, with his wife and two daughters, and being denied an exit visa until he agrees to drop a legal case against his former club, Al-Jaish, over his claim of almost two years unpaid wages.
In a personal letter to Blatter, FIFPro Secretary General, Theo van Seggelen, calls for FIFA's urgent intervention.
Van Seggelen said: "FIFPro insists that Belounis be allowed to leave Qatar and receive his wages immediately."
At the very least, Van Seggelen added, he should be freed, able to play for a new club and be guaranteed that he can claim his unpaid salary in a case before FIFA's Dispute Resolution Chamber (DRC).
FIFPro board member Mads Oland and representatives from the International Trade Union Confederation (ITUC) are set to travel to Qatar soon to meet the Belounis family and force the issue of their release.
According to FIFPro, the Belounis case is a violation of basic human rights.
It said Belounis joined Qatari side Al-Jaish in 2007. He later extended his contract until June 2015 but from November 2011 the club stopped paying his salary.
The club, it said, then put him under pressure to terminate his contract and sign a document confirming he was owed nothing by Al-Jaish.
Belounis refused, concerned his signature would invalidate any claim.
In Qatar - hosts of the 2022 World Cup - an employee is unable to leave the country without an exit visa which the employer has to obtain.