The U.N. has called on the guarantors of the Sochi deal, Turkey and Russia, to protect northwestern Syria's Idlib from a new assault by the Bashar Assad regime, warning against an unfolding humanitarian crisis in the province.

"I appeal in particular to the Astana guarantors – and to the Russian Federation and Turkey, especially, as the signatories of the September 2018 Memorandum of Understanding on Idlib – to stabilize the situation without delay," U.N. Secretary-General Antonio Guterres told reporters at the U.N. Tuesday.

He also emphasized that there could be no military solution to the Syrian crisis. "It was clear at the start and it remains clear more than eight years later that the solution must be political," Guterres said.

The Sochi agreement was reached in September between Ankara and Moscow, envisaging the preservation of the cease-fire in Idlib, with the withdrawal of heavy arms and radical groups from the region. Idlib, the last opposition enclave in Syria, had a prewar population of 1.5 million which 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.

The Assad regime's attacks on Idlib also prompt fears of a new refugee wave as international organizations have warned against a humanitarian crisis.

The U.N. Under-Secretary-General for Humanitarian Affairs and Emergency Relief Coordinator Mark Lowcock told the U.N. Security Council Tuesday that "a humanitarian disaster" is unfolding in Syria's Idlib, noting that estimated 330,000 people have been forced to flee their homes and more than 230 civilians have died since Bashar Assad regime troops began pushing into Idlib on April 30.

Lowcock said the World Health Organization (WHO) has confirmed that 26 health care facilities in northwestern Syria have been attacked since late April and stressed that attacking civilians and civilian installations like hospitals and schools is a violation of international law. "A number of partners now feel that supplying geographical coordinates to be given to the warring parties effectively paints a target on their backs," he said. "Some have drawn the conclusion that hospital bombings are a deliberate tactic aimed to terrorize," Lowcock added.

Time, strategy needed to clear HTS from Idlib

U.N. political chief Rosemary DiCarlo told the council that for Assad's ally, Russia, the presence of Hayat Tahrir al-Sham (HTS) radicals in Idlib, "is not tolerable" and "for Turkey, time is required to effectively isolate and address HTS' most hard-line fighters."

Following eight months of calm provided by the Sochi deal, the Assad regime intensified its attacks starting from April 26 under the pretext of fighting HTS militants holed up in Idlib.

Turkey's U.N. Ambassador Feridun Sinirlioğlu said the HTS problem needs to be addressed "with a more sophisticated and comprehensive long-term strategy, targeting its ideology and structure."

"Progress in the political process will be one of the key elements to this end," he said.

For over a year, the U.N. has been trying to form a committee to draft a new constitution for Syria, and Sinirlioğlu said finalizing an agreement is at a "critical stage."

Convening the committee "will be the first essential step of the international community's efforts towards a democratic Syria," he said.

Astana talks also support the establishment of the U.N.-backed constitutional committee in Syria to find a political solution. The planned constitutional committee will be tasked with carving out Syria's post-war constitution, which is seen as a stepping stone to elections in the war-torn country.

But Sinirlioğlu said the Bashar Assad regime's attacks in Idlib are clearly aimed "at the collapse of the political process."

"If the Idlib de-escalation area cannot hold," he warned, "prospects for a viable political solution will diminish considerably."

Earlier Sunday, Bashar Assad regime forces attacked a Turkish observation post in Idlib, Syria with no casualties; Turkey retaliated with heavy weapons, the Defense Ministry said in a statement.

Regime forces launched an artillery attack from Idlib region's Tall Bazan on Murak, where a Turkish observation post is situated, it added. It was the second intentional attack by the regime, according to the Defense Ministry, as last week Turkey's 10th observation post was "deliberately" targeted in a mortar attack in which three soldiers were wounded.