Tens of thousands of Syrians fled an intensifying Russian assault around Aleppo on Friday, and aid workers said they feared the city which once held two million people could soon fall under a full government siege.
Iran reported one of its generals was killed on the front line assisting government forces, direct confirmation of the role Tehran is playing along with Moscow in what appears to be one of the most ambitious offensives in five years of civil war.
The government assault around Aleppo, and advances in the south of the country, helped to torpedo peace talks this week in Geneva. President Bashar al-Assad's forces and their allies are making a new bid to achieve victory on the battlefield after Russia's intervention ended months of stalemate.
The last 24 hours saw government troops and their Lebanese and Iranian allies fully encircle the countryside north of Aleppo and cut off the main supply route linking the city - Syria's largest before the war - to Turkey. Ankara said it suspected the aim was to starve the population into submission.
Aleppo would be the biggest strategic prize in years for Assad's government in a conflict that has killed at least 250,000 people and driven 11 million from their homes.
Video footage showed thousands of people, mostly women, children and the elderly, massing at the Bab al-Salam border crossing. Men carried luggage on top of their heads, and the elderly and those unable to walk were brought in wheelchairs. Some women sat on the side of the road holding babies and awaited to be allowed into Turkey.
"It feels like a siege of Aleppo is about to begin," said David Evans, Middle East program director for the U.S. aid agency Mercy Corps, which said the most direct humanitarian route to Aleppo had been severed.
The leader of a prominent rebel group active in northwestern Syria confirmed that government-allied forces were tightening their grip on the northern Aleppo countryside, and that heavy Russian bombing carried on unabated.
NON-STOP RUSSIAN AIR STRIKES
"The Russian (air) cover continues night and day, there were more than 250 air strikes on this area in one day," Hassan Haj Ali, head of Liwa Suqour al-Jabal, a group that fights under the umbrella of the Free Syrian Army, told Reuters.
"The regime is now trying to expand the area it has taken control of ... Now the northern countryside (of Aleppo) is totally encircled, and the humanitarian situation is very difficult," he said.
Syrian state TV and a monitoring group said the army and its allies had seized the town of Ratyan north of Aleppo, building on gains made earlier in the week. Haj Ali said the town had not yet fallen, but that there were "very heavy battles".
The Syrian army and its allies broke a three-year rebel siege of two Shi'ite towns in Aleppo province on Wednesday, cutting off a major supply line from Turkey to Aleppo.
Aleppo, Syria's commercial hub, has been divided for years between a section under government control and areas that are in the grip of rebels. Much of Aleppo, including a UNESCO heritage old city, is largely in ruins.
Haj Ali said most of the fighters on the government side were "Iranian and from Hezbollah, or Afghan".
Iran's semi-official Tasnim news agency said Revolutionary Guard Corps Brigadier-General Mohsen Ghajarian has been killed in Aleppo province, as had six Iranian volunteer militiamen.
The five-year civil war pits a government led by Assad, a member of the Alawite sect derived from Shi'ite Islam, against a range of insurgents who are mainly Sunni Muslims, backed by Saudi Arabia, other Arab states and Turkey. Western countries have also lined up in opposition to Assad.
ARMY GAINS IN DERAA
Since 2014, the Sunni jihadist group Islamic State has run a self-proclaimed caliphate in eastern Syria and Iraq, under air assault from a U.S.-led coalition. Russia launched its own separate air campaign four months ago to aid its ally Assad, transforming the battlefield and tipping momentum his way.
But swathes of the country are still in the hands of armed rebels, including Islamic State in the east, Kurdish militia in the north, and a mosaic of groups in the west who have been the target of many of the Russian air strikes.
In addition to the advance in the north near Aleppo, Syria's government forces and allies made further gains in the southern province of Deraa, recapturing a town right outside Deraa city.
That advance could provide a more direct supply line for the army from Damascus and allow it to assert control over most parts of the city.
It has been backed by some of the heaviest Russian air strikes since it began its bombing campaign in September, a rebel spokesman in the area said.
Peace talks convened this week in Geneva were the first diplomatic attempt to end the war in two years but collapsed before they began in earnest. The opposition refused to negotiate while Russia was escalating its bombing and government troops were advancing.
NATO said Moscow's intensified bombing campaign undermined the peace efforts and warned Russia was creating tensions by violating the air space of Syria's neighbor Turkey, a NATO member which shot down a Russian warplane in November.
Russia has accused Turkey of preparing a military incursion into northern Syria. Ankara dismissed this as propaganda intended to conceal Russia's own "crimes". Aleppo was threatened with a "siege of starvation", and Turkey had the right to take any measures to protect its security, it said.
STRIKES UNDERMINE PEACE QUEST, NATO
Moscow says its targets in Syria are restricted to Islamic State and al Qaeda's Syrian branch, the Nusra Front, both of which were excluded from peace talks and officially anathema to the countries supporting the insurgents against Assad.
"Why did the opposition that left Geneva complain about the offensive in Aleppo, which is actually targeted against Jabhat al-Nusra (Nusra Front) and other radical extremist groups?" said Russia's ambassador to the United Nations in Geneva, Alexey Borodavkin.
"Jabhat al-Nusra is a terrorist organisation recognised by the U.N. Security Council. It's a branch of al Qaeda. The opposition should be happy that terrorists are defeated. But, on the contrary, they were disappointed and left negotiations."
That position is rejected by Western and Arab countries, which say most Russian strikes are against other opponents of Assad, not the banned groups.
"The intense Russia air strikes, mainly targeting opposition groups in Syria, is undermining the efforts to find a political solution to the conflict," said NATO Secretary-General Jens Stoltenberg.
Russian violations of Turkish air space were "causing increased tensions and ... create risks", he added.
Saudi Arabia, meanwhile, said it was ready to participate in separate U.S. ground operations against Islamic State. The United States welcomed the Saudi offer, although Washington so far has committed only to small scale operations by special forces units on the ground in Syria.
REUTERS