The groups said in a statement that any territorial advances by the army or by Iran-backed militias would end the fragile ceasefire brokered by Russia and Turkey.
came into force on Friday.
"The regime and its allies have continued firing and committed many and large violations," said the statement, which was signed by mainly moderate groups under the umbrella of the so-called Free Syrian Army (FSA).
According to the statement, the main violations are in an area northwest of Damascus where government forces and the Iran-backed Lebanese Hezbollah group have been trying to advance.
Rebels say the army is seeking to recapture the area.
Russia and Turkey announced the ceasefire last week, the third truce over the last 24 months that seeks to end the war.
The agreement calls for talks between President Bashar al Assad's government and the opposition, to be held in Kazakhstan this month.
On Saturday, the UN Security Council adopted a
resolution to endorse the ceasefire.
Nearly 50,000 people died in the Syrian conflict during 2016, according to the Britain-based Syrian Observatory for Human Rights.
Source : https://news.sky.com/story/syrian-rebels-threaten-to-boycott-peace-conference-10716400