With the assistance of Russian airstrikes, President Bashar Assad's forces are pressing ahead with a major offensive around the northern city of Aleppo, a development that has sent another wave of Syrian civilians seeking refugee in neighboring Turkey.

Aleppo was Syria's largest city and an economic hub before the war, though many civilians have fled as it has become one of the most contested battlegrounds. For the past several years, government troops have controlled the western part and a patchwork of rebel forces have held the east.

The government forces began their push in recent months after Russia's military intervened on Assad's behalf. The Syrian army has cut a crucial rebel supply line to Turkey. This has stoked fears that the army is looking to surround the rebels and impose a siege on them and the hundreds of thousands of civilians in that part of the city.

The Aleppo fighting also contributed to suspension of Syrian peace talks in Geneva on Thursday. The Assad government has little incentive to negotiate at a time when it's making gains on the battlefield, and the rebels argue that Assad is not serious about negotiations.

The government's battlefield momentum elated its supporters. Residents in the towns of Nubol and Zahraa, near Aleppo, rejoiced as Syrian troops entered. The two Shiite towns were long cut off from other government territories. They had survived a siege that lasted three years, depending on government airdrops of food and supplies and rare transactions with a neighboring Kurdish enclave. The Syrian government pointed to conditions in these areas as a motivation for the current offensive.

But those on the losing side have begun to flee. A video posted by the Shahba Press Agency, which is aligned with the opposition, showed what aid agencies numbered at thousands of men, women, children and elderly massing at the Turkish border, some still running with their possessions. Local journalists say most of the families had already been displaced by government bombardments, and for years had been living in camps near the northern border.

Activists say those camps were long considered a safe zone for their proximity to Turkey, and to its Patriot missile systems deployed by NATO allies in 2013. Last year, the US and Germany decided to pull their Patriot systems from Turkey. Since then, hostilities have increased along the border. In late November 2015, Turkey's air force shot down a Russian jet, one of two crafts which it said violated its airspace. Moscow denies the planes crossed into Turkey or were given warnings.

And this week, activists published photos showing displaced persons' camps that had been abandoned in northern Syria. Cushions were scattered and tents were still smoldering in the wake of alleged government and Russian bombardments.

The United Nations condemned the airstrikes north of Aleppo and called for safe passage for civilians and aid workers. Turkey, under pressure to stem the flow of migrants to Europe, has kept its border shut in the face of the growing crowds.

Aid agencies say the loss of the main Aleppo-Turkey route has severely hampered aid deliveries to rebel-held parts of Aleppo and surrounding areas. Mercy Corps says its operations in northern Syria have effectively been cut in half, leaving hundreds of thousands of people cut off from food aid.

A rebel offensive in Aleppo in the summer of 2012 split the city in half. The government responded with airstrikes and crude barrel bombs. The government-held side in the west comes under fire from mortars and improvised rockets. Residents on both sides face regular power and water shortages.

Aleppo's rebel half in the east is not fully encircled — so far. Residents still have a path to Turkey through Idlib province to the west. But that is also under threat.

Syrian state media and the Syrian Observatory for Human Rights say that government forces and allied Shiite militiamen overran two more towns north of Aleppo in the past two days. Those forces are edging closer to squeezing off the rebels' last lifeline in and out of Aleppo.

Aleppo journalist Mohammed al-Khatieb told NPR that rebel reinforcements have begun shoring up defenses of the last outlet to Turkey. But he offered a sober assessment of the future.

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