Syrian rebel groups say they are suspending participation in the preparations for peace talks planned by Russia and Turkey for later this month.
A statement, signed by a number of groups, cited “many and large violations” of the ceasefire by the Syrian government as the reason.
Turkey and Russia brokered the ceasefire deal last Thursday, and it has mostly held since then.
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,”the statement issued on Monday said.
“As these violations are continuing, the rebel factions announce… the freezing of all discussion linked to the Astana negotiations,” it said.
The groups highlighted fighting in the rebel-held region of Wadi Barada, north-west of Damascus, which they say has been subjected to almost-daily bombing raids and bombardment by Syrian forces and their Hezbollah allies.
The area does not fall under the ceasefire agreement, given the presence of Jabhat Fateh al-Sham (JFS), a jihadist group which was excluded from the deal.
Wadi Barada contains a major spring, which supplies water to millions of residents in the capital. The government has accused the rebels of polluting it with diesel, which they deny.
While the rebels “respected the ceasefire across the whole of Syria… the regime and its allies have not stopped shooting”, the rebels’ statement said.
The Syrian military has denied the allegations.
On New Year’s Eve, the UN Security Council unanimously backed Russian-Turkish efforts to end the fighting in Syria and to organise peace talks.
The ceasefire deal excludes the jihadists of so-called Islamic State (IS) and 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.