Syrian rebels are worried the negotiated ceasefire may collapse, given Russian airstrikes are still pounding rebel positions.

The attacks have slowed to a trickle as a result of the U.S.-Russian ceasefire, but occasional Russian airstrikes are still ongoing, Reuters reports.

The rebels, based in Riyadh, Saudi Arabia, wrote a letter to United Nations Secretary-General Ban Ki-Moon, informing him Russia conducted a total of 26 bombing raids Sunday against rebel groups. The rebels also alleged Russia used cluster bombs against civilians. Some of that activity took place in the provinces of Aleppo and Idlib.

In response, the Syrian government said terrorist groups had shelled Latakia province, though it added later the attacks came from an area where Jabhat al-Nusra regularly operates. The ceasefire notably does not apply to al-Nusra or Islamic State.

Russia, through the outlet Russia Today, said rebel groups violated the agreement nine times, with six of the breaches taking place near Damascus.

The fact that rebels have mostly kept their side of the bargain indicates the 97 disparate rebel groups in Syria opposed to the current regime seem to view the High Negotiations Committee in Riyadh as legitimate. The High Negotiations Committee, established in December, purports to represent the rebel groups on the international stage.

Russia meanwhile accused Turkey of violating the agreement Sunday, sayingTurkish forces moved into Syria and attacked Tal Abyad just over the border. Tensions between Russia and Turkey have remained high since Turkey shot down a Russian fighter jet after it reportedly violated Turkish airspace. North Atlantic Treaty Organization (NATO) Secretary-General Jen Stoltenberg told Turkey if Russia attacked, NATO would defend it under Article 5, which mandates if one country in NATO is attacked, other members must respond.

Adel al-Jubei, the foreign minister for Saudi Arabia, bashed both the Russians and the Syrian government, led by President Bashar Assad, for failing to fully stick to the terms of the ceasefire. The ceasefire is technically not a formal agreement, but rather more of a deal. Still, most groups consider the deal to be holding reasonably well in spite of the violations.

The next round of peace negotiations is schedule for March 7.

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