Syrian rebel groups say they are suspending participation in the preparations for peace talks planned by Russia and Turkey for later this month.
In a statement, the rebels cited “many and large violations” of the ceasefire by the Syrian government as the reason for their move.
Turkey and Russia brokered the ceasefire deal that has mostly held since last Thursday.
The peace talks are due to be held in Astana, Kazakhstan.
“The regime and its allies have continued firing and committed many and large violations,” said the statement signed by a number of Syrian rebel groups on Monday.
“As these violations are continuing, the rebel factions announce… the freezing of all discussion linked to the Astana negotiations,” they said.
The statement highlighted fighting in the rebel-held region of Wadi Barada near Damascus, which they allege has been subjected to almost-daily bombing raids. In recent weeks the rebels have accused the government and its allies of trying to recapture the area, which supplies water to the capital.
While the rebels “respected the ceasefire across the whole of Syria… the regime and its allies have not stopped shooting”.
The military has denied the allegations which the rebels say jeopardise the entire peace process.
“Any [advance] on the ground goes against the agreement and if things don’t return to how they were before, the accord will be considered null and void,” the rebel statement said.
On New Year’s Eve, the United Nations Security Council voted unanimously to welcome the Russia-Turkey efforts to end the Syrian fighting and to organise peace talks.
The new ceasefire deal applies across Syria but excludes the jihadists of so-called Islamic State (IS) and Jabhat Fateh al-Sham (JFS), and the Kurdish YPG militia.
Who is included in the truce agreement?
On the one side, Syrian government forces, allied militias and the Russian military.
On the other, the FSA plus several other groups.
The Russian defence ministry named seven “moderate opposition formations” included in the truce as Faylaq al-Sham, Ahrar al-Sham, Jaysh al-Islam, Thuwwar Ahl al-Sham, Jaysh al-Mujahidin, Jaysh Idlib and Jabhah al-Shamiya.
Ahrar al-Sham, which said it had “reservations” about the deal, and Jaysh al-Islam are Islamist groups that Russia has previously described as terrorist organisations.
Who is not included?
IS and JFS and the groups affiliated to them are not part of the agreement, according to the Syrian army.
JFS said on Friday it would continue to fight President Assad, with a spokesman saying the political solution under the truce would “reproduce the criminal regime”.
Members of the group are currently operating as part of a rebel alliance that controls Idlib province.
The FSA also said the deal did not include the Kurdish Popular Protection Units (YPG).
The militia, which has captured large swathes of north-eastern Syria from IS with US support, is designated a terrorist organisation by Turkey.
The truce is nominally nationwide, although it really only covers the areas where the sides who have signed up have a presence – western Syria.
Map showing control of Syria and Iraq (28 December 2016)