The decision of the dozen rebel groups threatens the process to begin in late January in Astana, led by Russia and Iran, support of the regime, and Turkey, the support of the rebels.
In a statement issued on Monday night, these rebel groups accused the Bashar al-Assad regime of breaking the ceasefire, in effect since Dec. 30.
Among the groups that signed the text are the Islamist rebels Jaij al-Islam and Faylaq al-Rahman, present in Damascus, as well as the Sultan Murad group supported by Turkey, and Jaij al Ezza, active in the province of Hama.
- Al Qaeda's former branch chiefs are killed -
Meanwhile at least 25 members of the Fateh al-Cham Front group, including several chiefs, were killed in an air strike on Tuesday, according to the OSDH.
The OSDH added that the attack on the Fateh al-Sham Front aimed at "one of the most important headquarters of the group" in Syria, but could not specify who did if the international coalition led by the United States or Russia and its Syrian ally.
"At least 25 members of the group, including their leaders, were killed in the town of Sarmada, where they were in a meeting," OSDH director Rami Abdel Rahmane told AFP.
The Fateh al-Sham Front accused the international coalition of having carried out the attack and referred to "more than 20 dead" in a message on the crypt-telegram application.
According to an AFP correspondent, several bombings targeted other sites in Sarmada, including a prey in the hands of the Fateh al-Sham Front.
This jihadist organization is outside the truce in force in Syria since Thursday and is sponsored by Russia, ally of the Syrian regime and Turkey, support of the insurgents.
The province of Idleb is largely in the hands of Fateh al-Sham and allied rebel groups.
The regime's offensive, supported by fighters of the Lebanese Shi'ite movement Hezbollah, continued on Tuesday in Wadi Baradi, a rebel-controlled region 15 km from Damascus, according to the OSDH.
This sector is strategic since it houses the main sources of supply in drinking water for the four million inhabitants of the capital and its surroundings.
According to the OSDH, the troops of the regime use helicopters and carry out artillery shots. On Monday they advanced to the vicinity of Ain al-Fige, an important source of water.
The Syrian government accuses the rebels of damaging infrastructure, in particular, "contaminating with diesel" water supplies and cutting off the supply network to Damascus.
The rebels respond that it is the bombings that damage the facilities, disrupting the supply since December 22.
- 'Critical phase' -
OSDH director Rami Abdel Rahman said the truce is in a "critical phase" and threatens to "collapse" completely if Russia and Turkey do not intervene to rescue it.
OSDH reported on other breaches of the truce in Syria, in particular, air strikes against Jan Seijun, in Idleb province which is largely controlled by the rebels.
Russia, which works on the front line, won a compromise support from the UN Security Council on Saturday for its peace plan.
In full political transition before the assumption of Donald Trump, the United States, which supports opposition to the Asad regime, was not associated with this initiative, for the first time since the start of the war in March in 2011.
Instead, it is the first time that Turkey, which approached Russia, is participating in an agreement.
Military planes from the US-led international coalition supported Turkish forces last week on a mission near Al Bab in Syria, without using weapons, the Pentagon reported.
The support mission unveiled on Tuesday seems to account for an improvement in relations between the two partners. "Last week there was a request ... for air support when some Turkish forces were attacked, and there were coalition flights at that time," said Pentagon spokesman Peter Cook.
The Astana negotiations will precede negotiations scheduled for February in Geneva.
In the past, no intercessional dialogue allowed to begin a process of conflict resolution that left more than 310,000 dead and millions of displaced.