Beirut (AFP) - Thousands of Syrians braved the freezing cold at Turkey's border after fleeing a regime assault that threatens a new humanitarian disaster, as Damascus warned Riyadh and Ankara not to send in troops.
The government said any uninvited foreign soldiers who enter Syria would go home "in a wooden coffin", following reports that Saudi Arabia and Turkey, which support rebel forces, could deploy troops.
Tens of thousands of people have fled fierce fighting as government forces backed by Russian air strikes advanced this week against rebels, severing the opposition's main supply route into the northern metropolis of Aleppo.
Turkey's Oncupinar border crossing, which faces Bab al-Salama inside Syria, remained closed Saturday, an AFP correspondent said.
The United Nations said some 20,000 people had gathered at Bab al-Salama but the governor of Turkey's Kilis border province, Suleyman Tapsiz, said at least 70,000 may head for the frontier.
Tapsiz said the displaced were being accommodated in eight camps on the Syrian side and that Turkey -- already home to 2-2.5 million Syrians -- was also able to take care of 30-35,000 refugees inside Syria.
Turkish Foreign Minister Mevlut Cavusoglu said his country would keep its "open border policy" for Syrian refugees.
"We still keep this open border policy for these people fleeing from the aggression, from the regime as well as air strikes of Russia," he said.
"We have received already 5,000 of them; another 50,000 to 55,000 are on their way and we cannot leave them there."
According to the Britain-based Syrian Observatory for Human Rights, around 40,000 Syrian civilians have fled the regime offensive.
- 435 killed -
"Thousands have been sleeping in the open, in fields and on roads," on the border and in the nearby Syrian city of Azaz, said Mamun al-Khatib, director of the Aleppo-based pro-rebel Shahba Press news agency.
"And because the main rebel supply route between Aleppo and Turkey has been cut, the price of oil, foodstuffs and baby milk has shot up in the north of Aleppo province," he added.
The Observatory said 435 people have been killed since the regime offensive began on Monday, including 71 civilians, most of whom had died in Russian air strikes.
Also killed were 124 regime forces, 90 jihadists from Al-Qaeda's Syria affiliate Al-Nusra Front and 150 other rebels, it said.
Riyadh on Thursday said it would "contribute positively" if the US-led coalition against the Islamic State (IS) jihadist group in Syria decides on ground action.
Russia, a key ally of the Damascus government, accused Turkey of "preparations for an armed invasion" of Syria, a claim that Ankara dismissed.
Syria's Foreign Minister Walid Muallem issued a stiff warning against any such move.
"Any ground intervention on Syrian territory without government authorisation would amount to an aggression that must be resisted," he said.
"Let no one think they can attack Syria or violate its sovereignty because I assure you any aggressor will return to their country in a wooden coffin, whether they be Saudis or Turks," he added.
- Iran warning -
The head of the elite Revolutionary Guards of Iran, another key Syrian regime ally, mockingly said Saudi Arabia would not dare send in ground forces.
"I don't think they would dare do that... If they do, they will inflict a coup de grace on themselves," Major General Ali Jafari said.
Turkey last faced such an influx in 2014 when 200,000 refugees fled the Syrian Kurdish town of Kobane over three days as IS and Kurdish fighters battled for control of it.
Trucks were seen on Friday carrying tent parts to the refugee camp near the border gate on the Turkish side, and at least four more were seen returning to Turkey after delivering food in Syria.
More than 260,000 people have been killed in Syria's conflict and more than half the population has been displaced.
Also on Saturday, the mother of President Bashar al-Assad, Anissa Makhlouf al-Assad, died at the age of 86, state media reported.
She was the widow of president Hafez al-Assad, who ruled the country with an iron fist, but she kept a low profile and was rarely mentioned in the media when her husband was in power or during the country's nearly five-year war.