Millions of civilians that took shelter in northwestern Syria's Idlib are under serious threat amid a looming humanitarian catastrophe, a senior U.N. adviser said Thursday.

"We heard firsthand reporting from our U.N. colleagues based in the region about the increasingly horrific brutality in recent weeks by all parties involved in the de-escalation area in northwestern Syria, which has caused significant civilian causalities and the displacement of hundreds of thousands of people. Three million people in Idlib need protection. A humanitarian catastrophe is a grave danger if the violence does not cease," Najat Rochdi, a senior humanitarian adviser to the United Nations special envoy for Syria, said in a statement.

Following eight months of calm provided by the Sochi deal reached between Ankara and Moscow, the regime has intensified its attacks since April 26 under the pretext of fighting Hayat Tahrir al-Sham (HTS) militants holed up in Idlib. The intermittent attacks and bombardments have killed, wounded and displaced thousands. Residential areas have been destroyed by indiscriminate attacks, while numerous educational facilities, health facilities and residential areas collapsed or have become unusable after being targeted by bombs.

Rochdi said the involved parties should not set aside their obligations to civilians with the pretext of fighting terrorism.

"Combating terrorism does not absolve any party from its legal obligations under international humanitarian law to prevent attacks against civilians and civilian infrastructure. All warring parties must uphold their obligations under international law and immediately stop attacks on civilians and civilian infrastructure, including medical facilities, schools, markets and places of worship, which may amount to war crimes," he said.

Idlib is the opposition's last enclave. Its 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. Tens of thousands of Syrians trapped in other parts of the country were evacuated there under various cease-fire agreements.

Stressing that people who remain displaced in northeastern al-Hol camp need urgent protection and assistance, Rochdi said the 91 percent of the 72,000 people in the camp are women and children, with around 65 percent under the age of 12.

"All children, including those suspected of being associated with armed groups and/or terrorist organizations, are entitled to special care and protection under international humanitarian law and international human rights law, including the Convention on the Rights of the Child. These children are victims, and must be treated first and foremost as such," he said.

Rochdi also emphasized that the situation for 29,000 people in the Rukban refugee camp on the border with Jordan remains critical due to a desperately short supply of basic goods and services.

"They last received assistance more than four months ago. I call again today on member states with influence to facilitate the immediate delivery of assistance to people in Rukban and to support the U.N.'s request to assist those transiting to collective shelters and to people who have returned to areas of origin," he said.