A regime offensive in Syria's last rebel enclave has caused one of the biggest waves of displacement in the nine-year war, with tensions spiking between Ankara and Damascus following a deadly exchange of fire.
Weeks of intensive aerial bombardment and a bruising ground offensive have emptied entire towns in northwest Idlib and sent huge numbers fleeing north towards the Turkish border.
"Since 1 December, some 520,000 people have been displaced from their homes, the vast majority -- 80 percent -- of them women and children," said David Swanson, spokesman for the United Nation's humanitarian coordination office, OCHA.
The exodus, coinciding with a biting winter, is one of the largest since the 2011 start of a conflict that has seen more than half of Syria's pre-war population of 20 million displaced.
"This latest displacement compounds an already dire humanitarian situation on the ground," Swanson said.
He said the UN was alarmed by the plight of more than three million people -- half of them displaced by violence elsewhere -- who live in Idlib province and surrounding areas.
Government troops and militia forces backed by Russian and other allied forces have in recent weeks upped the pressure on the jihadist-dominated region.
Mohammad Bahjat, 34, said he and his family were escaping deadly bombardment for the third time in days.
"We fled in the middle of the night and have no idea where to go," he told AFP, sitting in the front of a pickup truck next to his wife and three small boys.
"You never know when a rocket or shell is going to hit."
- Three million displaced -
Regime forces also exchanged deadly fire with Turkish troops on Monday, an escalation UN chief Antonio Guterres called "extremely worrying".
Regime shelling of Turkish positions in Idlib killed at least five Turkish soldiers and three civilians, Ankara said.
The Syrian Observatory for Human Rights reported that retaliatory fire from Turkey killed at least 13 Syrian government troops.
President Recep Tayyip Erdogan on Tuesday told his Russian counterpart Vladimir Putin that Ankara would respond "firmly" to any new Syrian attack.
Syria's military command said the presence of Turkish forces was "illegal and a flagrant act of aggression", vowing to respond to any Turkish attack on its forces, state news agency SANA reported.
Guterres, speaking at UN headquarters, appealed for "a cessation of hostilities".
But US Secretary of State Mike Pompeo backed Washington's NATO ally, calling the Syrian mortar attack "a grave escalation" and saying US officials "fully support Turkey's justified self-defence actions in response".
Syrian forces have retaken dozens of villages and some major towns -- including the former rebel bastion of Maaret al-Numan -- and continue to advance, pushing displaced populations ever closer to the Turkish frontier.
Erdogan accused Damascus of driving "innocent and grieving people in Idlib toward our borders".
Turkey, which already hosts more than three million displaced Syrians, is keen to stop another mass influx.
But in the town of Hazano, an AFP correspondent saw dozens of small trucks, vans and tuk-tuks piled high with belongings crawling along a main road.
They advanced bumper-to-bumper, carrying men, women and children, along with blankets, carpets, washing basins and furniture.
Almost 300 civilians have been killed since mid-December in bombardment on the Idlib region, says the Britain-based Observatory.
Syrian forces seized much ground in the enclave's south and are also attacking from Aleppo province to the east.
- 'War crimes' -
UN experts said Tuesday that the latest attacks could be war crimes.
The UN's Independent International Commission of Inquiry on the Syrian Arab Republic cited reports of "attacks on civilian infrastructure, including schools, markets, and medical facilities" in the last two weeks.
"The deliberate and systematic targeting of hospitals follows a pattern already documented by the Commission, and may amount to war crimes," it said in a statement.
Doctors Without Borders (MSF) also condemned attacks against healthcare infrastructure.
Several hospitals "have been hit and have been either partially or fully destroyed in... the space of just a few months" said Cristian Reynders, the NGO's project coordinator for northern Idlib.
The spiking tensions between Ankara and Damascus have raised fears of yet more violence.
Monday's incident was their deadliest clash since Ankara sent troops to Syria in 2016.
The an incident that further tested the uneasy coordination between Russia and Turkey -- the two main foreign brokers in the Syrian conflict.
Under a deal reached last year, Russia was to prevent Damascus from launching a devastating operation in the densely populated region.
Turkey was to contain the extremist groups running the region, but instead, the forces it supports have been sidelined by Syria's former Al-Qaeda affiliate, Hayat Tahrir al-Sham, which dominates Idlib.