TAL RIFAAT, Syria - Military helicopters pounded a rebel-held district of Aleppo on Saturday and armored units positioned themselves for an onslaught that could determine the fate of Syria's biggest city, opposition sources said.
Turkey, once a friend but now a fierce critic of the Syrian government, joined growing diplomatic pressure on President Bashar Assad, calling for international steps to deal with the military buildup.
The Syrian Observatory for Human Rights, an opposition group that collects information on the 16-month-old uprising against Assad, reported helicopter attacks on Aleppo's central Salaheddine district and fighting elsewhere in the city.
"Helicopters are participating in clashes at the entrance of Salaheddine district and bombarding it," the group said in an emailed statement. "There are also violent clashes at the entrances to Sakhour district."
One opposition activist said he had seen tanks and armored carriers heading for Salaheddine.
On the approaches to Aleppo from the north, many villagers were still shopping or tending their fields. But fighters from the rebel Free Syrian Army were also in evidence, while a military helicopter clattered overhead in the distance.
One man in his 40s, carrying his family on a motorcycle, said he was fleeing the fighting in Aleppo and heading to the town of Azaz near the Turkish border.
"We are living in a war zone," he told Reuters. "I and my relatives are just going back and back and forth, trying to stay away from the fighting. We left Aleppo when we saw smoke and helicopters firing."
On the road south from the Turkish border to Aleppo, rebel soldiers had set up checkpoints bearing the sign: "This is an FSA checkpoint. May God protect you."
The battle for the city of 2.5 million people is seen as a crucial test for a government that has committed major military resources to retaining control of its two main power centers, Aleppo and the capital Damascus.
Military experts believe that while Assad's more powerful military will overcome the rebels in Aleppo and other major cities, it risks loss of control in the countryside because the loyalty of large sections of the army is in doubt.
"Assad's forces are likely to achieve a tactical victory that will represent a setback to opposition forces and allow the regime to demonstrate its military dominance," said analyst Ayham Kamel of the Eurasia Group. But he said the rebels were getting stronger while the military was on the wane.
Three rebel fighters were killed in clashes before dawn yesterday in Aleppo, the Observatory said. It said 160 people were reported killed in Syria on Friday, adding to an overall death toll of around 18,000 since the uprising began.
Fighting was reported in other towns across Syria: Deraa, the cradle of the revolution, Homs, the scene of some of the bloodiest combat, and Hama, where a revolt against Assad's father in the early 1980s was suppressed with thousands of deaths.
Turkish Prime Minister Tayyip Erdogan said late Friday that international institutions needed to work together to address the military assault on Aleppo and Assad's threat to use chemical weapons against external enemies.
Russia said international support for Syrian rebels would lead to "more blood" and the government could not be expected to willingly give in to its opponents.
Foreign Minister Sergei Lavrov, whose country has vetoed three UN Security Council resolutions intended to increase pressure on Assad, said Western and Arab nations should exert more influence on rebels to stop fighting.
Russia also said it would not allow searches of Russian-flagged ships under new European Union sanctions governing vessels suspected of carrying weapons to Syria.
UN Secretary-General Ban Ki-moon demanded that the government state it would not use chemical weapons "under any circumstances."
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