BEIRUT/ONCUPINAR, Turkey (Reuters) - The Syrian army advanced towards the Turkish border on Monday in a major offensive backed by Russia and Iran that rebels say now threatens the future of their nearly five-year-old insurrection against President Bashar al-Assad.
Iranian backed-militias played a key role on the ground as Russian jets intensified what rebels call a scorched earth policy that has allowed the military back into the strategic northern area for the first time in more than two years.
"Our whole existence is now threatened, not just losing more ground," said Abdul Rahim al-Najdawi from Liwa al-Tawheed, an insurgent group. "They are advancing and we are pulling back because in the face of such heavy aerial bombing we must minimise our losses."
The Russian-backed Syrian government advance over recent days amounts to one of the biggest shifts in momentum of the war, helping to torpedo the first peace talks for two years, which collapsed last week before they had begun in earnest.
The Syrian military and its allies were almost five km (3 miles) from the rebel-held town of Tal Rafaat, which has brought them to around 25 km (16 miles) from the Turkish border, the rebels, residents and a conflict monitor said.
The assault around the city of Aleppo in northern Syria has prompted tens of thousands to flee towards Turkey, already sheltering more than 2.5 million Syrians.
In the last two days escalating Russian bombardment of towns northwest of Aleppo, Anadan and Haritan, brought several thousand more, according to a resident in the town of Azaz.
Aleppo, Syria's largest city before the war with 2 million people, has been divided for years into rebel and government-held sections. The government wants to take full control, which would be its biggest prize yet in a war that has already killed at least 250,000 people and driven 11 million from their homes.
Rebel-held areas in and around Aleppo are still home to 350,000 people, and aid workers have said they could soon fall to the government.
Turkish President Tayyip Erdogan was quoted at the weekend as saying Turkey was under threat, and Ankara has so far kept the border crossing there closed to most refugees.
There are now around 77,000 refugees taking shelter in camps on the Syrian side of the Turkish border, Turkish Deputy Prime Minister Numan Kurtulmus said on Monday. He said that a worst-case scenario could see as many as 600,000 at Turkey's border.
After around a week of heavy Russian air strikes, Syrian government troops and their allies broke through rebel defences to reach two Shi'ite towns in northern Aleppo province on Wednesday, choking opposition supply lines from Turkey.
German Chancellor Angela Merkel said she was "appalled" by the suffering of Aleppo, blaming primarily Russian bombing and suggesting it violated a U.N. Security Council resolution Moscow signed in December.
Kerem Kinik, Vice President of the Turkish Red Crescent, told reporters at the Oncupinar border crossing that Syrians were fleeing Russian strikes in panic. The closure of the road to Aleppo risked a much larger scale repeat of crises in Ghouta, a besieged Damascus suburb, or even Madaya, a blockaded town were residents have starved.
"The route to Aleppo is completely closed and this is a road that was feeding all the main arteries inside Syria. Unless this is reopened, you will see Aleppo falling day by day into a similar situation as in Madaya and Ghouta and you will see a deepening humanitarian crisis," he said.
"They are hitting any vehicles that are on the move, they are hitting aid trucks," he added. "We really urge that the Russian attacks on Azaz and Aleppo should stop, because if there is such a policy to clear this area of all human beings... then we may not be able to cope with the influx."
The Syrian army's success in opening a route to the Shi'ite towns of Nubul and Zahraa enabled it to cut a highway that linked rebel held areas in the northern countryside with the eastern part of Aleppo held by insurgents since 2012.
The latest gains by the Syrian government bring it to the closest point to the Turkish border since August 2013, the Syrian Observatory for Human Rights said.
The capture of the towns of Mayer and then Kafin, just north of Nubul and Zahraa, in the past 24 hrs have opened the road towards Tal Rifaat, the next focus of the army assault. The capture of that would leave only the town of Azaz before the Turkish border itself.
The loss of Azaz, just a few miles from the Bab al Salama border crossing, would virtually wipe out insurgents from one of their main strongholds in northwest Syria, though they still control much of nearby Idlib province.
Russian bombing has for weeks targeted rebel routes to the main border crossing, once a major gateway from Europe and Turkey to the Gulf and Iraq, lately a lifeline for rebel-held areas in Idlib and Aleppo provinces.
The army's advance has also been indirectly helped by Kurdish-led YPG fighters who control the city of Afrin, southwest of Azaz. They have seized a string of villages in recent days, rebels and the Observatory said.
In a multi-sided civil war that has drawn in global and regional powers, the Kurds are the strongest allies on the ground in Syria of a U.S.-led coalition bombing Islamic State in eastern Syria and northern Iraq. Turkey supports other rebel groups against Assad and is hostile to the Syrian Kurds, which it sees as allies of its own Kurdish separatists.
Russia joined the war last year with air strikes that it says are aimed at Islamic State, but which Turkey, Arab states and the West say are aimed mostly at other opponents of Assad.
Four months of Russian air strikes have tipped momentum Assad's way. With Moscow's help and allies including Lebanon's Hezbollah and Iranian fighters, the Syrian army is regaining areas on key fronts in the west.
United Nations investigators called for new sanctions on Syrian officials as well as leaders of the two most hardline rebel groups, Islamic State and the Nusra Front, accusing the three of mass killings, torture and disappearances of civilians in custody.
Speaking in Ankara, Merkel, under fire at home over the refugee crisis, said Europe needed to follow up quickly on pledges of aid to help Turkey cope with the Syria exodus, and also urged Ankara to act fast to improve the situation for refugees.
Additional reporting by Nick Tattersall in Istanbul, Yesim Dikmen and Ercan Gurses in Ankara and Stephanie Nebehay in Geneva; writing by Suleiman al-Khalidi and Philippa Fletcher; editing by Peter Graff and Pravin Char