Civil defence members look for survivors after an airstrike on the rebel-held Old Aleppo, Syria April 16, 2016. REUTERS/Abdalrhman Ismail

BEIRUT (Reuters) - Syrian rebel groups announced a new battle against government forces on Monday, a sign of escalating violence that has undermined a ceasefire deal and threatens to derail U.N.-led peace talks.

The groups, which included factions fighting under the banner of the Free Syrian Army and powerful Islamist Ahrar al-Sham, said in a statement they would respond “with force” against any government forces that fired on civilians.

The ceasefire deal has been strained to breaking point by escalating fighting, particularly around the divided city of Aleppo, with each side blaming the other for the escalation that has underlined the huge challenge facing the peace talks. Heavy air strikes have also been reported north of Homs.

Senior opposition figures who have asked not to be quoted have said pressure is growing for a speedy decision to leave talks being convened by U.N. envoy Staffan de Mistura in Geneva, with no sign of progress towards discussion of a political transition sought by President Bashar al-Assad’s opponents.

The statement announced the “formation of a joint operations room to begin the battle... in response to violations by the army of Assad.” It gave no geographic location for the new battle.

Senior Syrian opposition negotiators on Sunday urged rebels to strike back against the Syrian army, accusing it of using a cessation of hostilities to gain ground, and cast doubt over whether they would continue Geneva peace talks indefinitely.

The cessation of hostilities agreement brokered by the United States and Russia came into effect on Feb. 27 with the aim of allowing peace talks to get underway. The agreement did not include Islamic State or the al Qaeda-linked Nusra Front.