Rebels fired mortars from a planned demilitarised zone in north-west Syria, days after they were meant to have withdrawn such weapons from the area, raising fears about the fate of a deal between Turkey and Russia to prevent a regime assault on the province.
The Syrian Observatory for Human Rights said rebels fired several "mortar shells at an army position in nearby Hama province, killing two soldiers" late on Saturday.
Rebels also shelled Aleppo province from other parts of the demilitarised area, with no casualties recorded, the UK-based monitor said.
"This is the first clear violation of the deal since the heavy weapons were withdrawn. This area is supposed to be clear of heavy weapons, including mortar shells," said Observatory head Rami Abdel Rahman.
"The entire reason for withdrawing from the buffer zone is that so mortars cannot be used because [their range] is roughly 15 km. Now shelling has killed two Syrian soldiers in Hama. That’s exactly what was not supposed to happen," said Joshua Landis, a Syria expert and head of the Center for Middle East Studies at the University of Oklahoma.
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Under a deal reached by opposition backer Turkey and regime ally Russia last month, rebels and militants alike were meant to have pulled all heavy arms from the horseshoe-shaped zone by October 10.
The deadline was reportedly met, with Ankara, rebel factions, and the Observatory all reporting that the area was free of heavy-duty weaponry.
However, on Sunday morning Syrian pro-government newspaper Al Watan said western parts of Aleppo province were being hit with "rocket fire and shelling with heavy weapons, which were supposed to be pulled out from the area".
The shelling comes a day before the deadline for extremist militants to withdraw from the area under the deal agreed between Russian President Vladimir Putin and Turkish President Recep Tayyip Erdogan in Sochi on September 17.
Syrian President Bashar Al Assad's threat to launch an all-out military offensive to retake Idlib, the last rebel-held province in the country after seven years of civil war, had sparked worldwide alarm over the fate of its roughly 3 million residents.
The demilitarised zone agreed by Russia and Turkey was intended to separate rebel and regime areas and would be monitored by the two sponsor countries.
The buffer includes parts of Idlib province and adjacent areas of Aleppo, Hama and Latakia provinces. Turkey, which backs a coalition of rebel groups in Idlib known as the National Liberation Front (NLF), reinforced its troop presence at observation posts around the province to enforce the deal. However, the majority of territory in the province is controlled by extremist groups, mainly the former Al Qaeda affiliate Hayat Tahrir Al Sham.
Elizabeth Tsurkov, a research fellow specialising in Syria at the Forum for Regional Thinking, said the presence of these groups in the demilitarised zone posed a greater threat to the Idlib deal than the shelling by rebels.
"The jihadist groups quietly removed heavy weapons from the demilitarised zone, as per the Sochi agreement, but they are also required from entirely withdraw from the territory," Ms Tsurkov said.
"It appears unlikely that the jihadist groups will simply agree to lose much of their territorial holdings, which are a source of power and income (through taxation), to appease Turkey."
She said Turkey would need to back up its diplomatic pressure on the extremists to abide by the deal "with a clearly communicated threat to use military force".
While the NLF welcomed the deal, HTS has made no comment. Hurras Al Deen, an even more hardline faction, rejected it. The NLF announced it had completed withdrawing its heavy weaponry from the buffer zone on October 8, and a day later the Observatory said HTS and "other less influential jihadist groups" also removed heavy arms from large parts of the demilitarised area.