BEIRUT (AP) — Syrian opposition groups allied with Turkey lobbed hundreds of missiles and artillery rockets at government posts in northwestern Syria on Tuesday, in retaliation for a deadly attack that killed dozens of their fighters a day earlier.

The renewed violence has undermined an already shaky cease-fire in place since March that aimed to quell military operations and government troop advances in the overcrowded rebel-held enclave.

The escalation also comes as relations between Russia and Turkey, who negotiated the cease-fire, show signs of strain over Ankara's increased military involvement in a region stretching from Syria to the Caucasus and the Mediterranean. Russia is a main supporter of Syrian President Bashar Assad's government.

U.N Special Envoy for Syria Geir Pederson appealed to Russia and Turkey to “contain the situation.” Meanwhile, Turkish Foreign Minister Mevlut Cavusoglu and his Russian counterpart, Sergey Lavrov, discussed the attack in Idlib as well as the situation in Nagorno-Karabakh and Libya in a telephone call on Tuesday, the Turkish Foreign Ministry said. It did not provide details.

Monday’s strike was the deadliest in Idlib since the Turkish-Russian-brokered truce there came into effect, raising fears that the truce could further fray. Some 1 million people were displaced by the last offensive inside the already packed enclave, home to over 3 million.

In retaliation, the Turkey-backed groups, operating under the umbrella of the National Front for Liberation, fired hundreds of artillery rounds and missiles since late Monday at government posts in territories adjacent to areas they control in Idlib and Aleppo provinces.

A spokesman for the NFL, Naji al-Mustafa, said the rebels' military retaliation targeted and killed Russian officers in southern Idlib, as well as Syrian soldiers working in the area.

The report could not be independently verified and there was no immediate comment from Russia or Syria.

The Britain-based Syrian Observatory for Human Rights recorded hundreds of projectiles lobbed by opposition fighters at nearly 30 government posts in different locations in southern Idlib, western Aleppo and the coastal province of Latakia. The Observatory said 12 Syrian soldiers and allied fighters were killed in the barrage.

The Monday airstrike on a rebel training camp near the border with Turkey killed more than 50 Turkish-backed fighters, according to one opposition spokesman, and wounded nearly as many, in one of the heaviest blows to the opposition’s strongest groups. The Observatory for Human Rights, which monitors the war in Syria, put the toll at 78 fighters dead and nearly 90 wounded. Videos circulating online showed the bodies of about a dozen men spread on the ground of an open space, wrapped in blankets.

The camp, operated by Faylaq al-Sham, an NFL faction, was hosting training sessions for new recruits. The NFL said a “large number” of fighters were killed, but declined to give details. It vowed retaliation and blamed Russia for the attack.

U.S. Special Representative for Syria James Jeffrey said the escalation in Idlib in violation of the March cease-fire deal is “dangerous” and threatens to prolong the conflict and deepen the Syrian people's suffering. Jeffrey said the UN-led political process is the only way to peace and stability in Syria.

“By continuing their quest for a military victory, the Assad regime and its Russian and Iranian allies are threatening the stability of the surrounding region,” he said in a statement Tuesday. “It is time for the Assad regime and its allies to end their needless, brutal war against the Syrian people.”

Russia and Turkey, although they support opposite sides in Syria’s nine-year conflict, have worked together to maintain a cease-fire in the last enclave of Syria’s rebels.

In the first official comment following the violence, the Turkish state-run Anadolu Agency said top Turkish presidential aide Ibrahim Kalin discussed the situation in Idlib with U.S. President Donald Trump’s National Security Adviser Robert O’Brien late Monday. The two men expressed concern over “recently increased attacks by the regime and its supporters,” according to Anadolu.

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Associated Press writer Suzan Fraser contributed to this report from Ankara, Turkey.