The Syrian Opposition and the Revolutionary Forces (SMDK) called on the international community on late Monday to take action against airstrikes and bombardment by the Bashar Assad regime and its backers in the Idlib de-escalation zone where hundreds of civilians were killed in recent weeks.

Stressing that the attacks in northwestern Syria are in violation of international treaties and agreements, an SMDK statement called on the United Nations take necessary steps to prevent attacks and to work for a political solution.

Being the opposition's last enclave, Idlib's prewar population of 1.5 million has swelled to around 3 million with new refugee waves after it was designated a "de-escalation zone" under the Astana agreement between Turkey, Russia and Iran in May 2017 to pave the way for a permanent political solution in Syria.

Turkey and Russia inked a deal for a buffer zone in September in Sochi to prevent a massive regime offensive on the Idlib region, near the Turkish border.

Following eight months of calm provided by the Sochi deal, the Assad regime intensified its attacks starting April 26 under the pretext of fighting Hayat Tahrir al-Sham (HTS) militants holed up in Idlib.

More than 540 civilians have been killed in regime and Russian airstrikes on northwest Syria since the end of April, the Syrian Observatory for Human Rights says. The United Nations says 25 health facilities in the region have been hit.

The SMDK's statement also drew attention to high civilian death toll in recent months and use of cluster bombs and barrel bombs in civilian areas by the Assad regime and its backers.

Many human right groups documented previously that the Syrian regime has used banned weapons multiple times on civilian areas. Despite the fact that the U.N. Security Council was urged numerous times to impose sanctions on the regime and its allies for atrocities and use of these weapons, no action has been taken so far.

In a latest report by the Syrian Network for Human Rights (SNHR) published in April, the regime has carried out at least 216 chemical weapons attacks, 217 cluster munition attacks and 19 incendiary weapons attacks on populated areas, while at least 77,146 barrel bombs have been dropped by regime helicopters and warplanes.

The regime has also been dropping phosphorus bombs, another banned weapon by international law, on Idlib. The use of white phosphorus is forbidden under international law.

Phosphorus, which poisons humans when inhaled, causes major damage to the brain and lungs and can lead to death. When used, white phosphorus creates a cloud of thick white smoke that can melt through the human flesh.

The Syrian civil war erupted in 2011 when the Assad regime responded harshly to protesters that poured onto the streets to demand more rights and freedom.

While the regime lost control of significant parts of the country in the first years of the civil war, it recently recovered significant swathes of territory with the backing of Russian and Iran and is now eyeing Idlib on the way to a "final victory" against the Syrian opposition.