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By Suleiman Al-Khalidi and Humeyra Pamuk BEIRUT/ONCUPINAR, Turkey (Reuters) - The Syrian army advanced toward 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. "They are advancing and we are pulling back because in the face of such heavy aerial bombing we must minimize 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.