BEIRUT: Syrian rebel groups said Saturday they would consider a cease-fire deal brokered by Russia and Turkey "null and void" if the Damascus government's forces and their allies continued to violate it.

Russia, which supports Syrian President Bashar Assad, has urged the United Nations to give its blessing to the fragile ceasefire, the third truce this year seeking to end nearly six years of war in Syria.

Clashes and airstrikes have persisted in some areas since the cease-fire began Friday, though the Syrian Observatory for Human Rights, a Britain-based opposition-aligned group, said Saturday the truce was still largely holding.

"Continued violations by the regime and bombardment and attempts to attack areas under the control of the revolutionary factions will make the agreement null and void," a statement signed by a number of rebel groups said.

It said government forces and their allies had been trying to press advances, particularly in an area northwest of Damascus.

Russia and Turkey, which backs the armed opposition to Assad, brokered the cease-fire agreement in the hope of preparing the way for peace talks in Kazakhstan in the new year.

In their statement, the rebels said it appeared the government and the opposition had signed two different versions of the cease-fire deal, one of which was missing "a number of key and essential points that are non-negotiable," but did not say what those were.

There has been confusion over which groups in the opposition are included in the ceasefire. ISIS, which has made enemies of all sides in the conflict, is not included.

The Syrian army said on Thursday the militant group formerly known as the Nusra Front was not part of the truce. However, several rebel officials said the group, which has been renamed Jabhat Fatah al-Sham, was also included in the cease-fire deal.