BEIRUT (AP) — The Syrian government launched an offensive Saturday to retake rebel-held neighborhoods in the nation’s commercial hub of Aleppo, unleashing artillery, tanks and helicopter gunships against poorly armed opposition fighters.

Yet after a day of fighting, the rag-tag rebel forces remained in control of their neighborhoods in Syria’s largest city, said activists, suggesting they had successfully fought off the government’s initial assault.

The international community has raised an outcry about a possible massacre in this city of 3 million but acknowledged there was little they could do to stop the bloodshed. The foreign minister of Russia, a powerful ally of Syria, said it was “simply unrealistic” for the Syrian regime to cede control.

The state-controlled al-Watan newspaper celebrated the assault with a banner headline proclaiming the fight for Aleppo “the mother of all battles.”

The rebels are estimated to control between a third and a half of the neighborhoods in this sprawling city, especially a cluster in the northeast around Sakhour neighborhood and in the southwest.

They began their attempt to wrest this key city from the government’s control a week ago. About 162 people have been killed, mostly civilians, according to the Britain-based Syrian Observatory for Human Rights, which does not include soldiers in its toll. Some 19,000 people have been killed since the uprising began in March 2011, estimated the group.

For Saturday, activists estimate that at least two dozen have died so far in the day’s fighting.

Local activist Mohammed Saeed said the rebels have managed to keep the regime’s tanks at bay so far with rocket-propelled grenades.

“The army hasn’t been able to take any neighborhoods yet, there are too many from the Free Syrian Army,” Saeed said, referring to the rebels.

He estimated that about 1,000 fighters had poured into the city in the past few days to take on the Syrian army, which had been massing forces around the city ahead of its attack.

By the end of Saturday, according to the Observatory, the government appeared to have pulled back from its ground offensive and was resuming its bombardment of various neighborhoods with artillery. Attack helicopters pounded rebel positions.

There were few details about the attack in the state press, although it issued a long list of victories across the country against the “terrorists,” as the rebels are referred to, a sign of widespread fighting.

The international community has expressed growing concern that there could be major bloodshed if Syrian troops retake Aleppo. But Western nations and their allies have found themselves powerless to prevent the situation from deteriorating despite a series of diplomatic efforts, including a cease-fire agreement that never took effect.

Kofi Annan, who brokered the agreement, expressed concern Saturday about the weapons buildup in Aleppo. “I remind the parties to the conflict of their obligations under international humanitarian law and human rights law, and urge them to exercise restraint and avoid any further bloodshed.”

“I am concerned about reports of the concentration of troops and heavy weapons around Aleppo, in anticipation of an imminent battle,” Annan said in a statement, according to Reuters.

“The escalation of the military build-up in Aleppo and the surrounding area is further evidence of the need for the international community to come together to persuade the parties that only a political transition, leading to a political settlement, will resolve this crisis.”

In a statement, the Arab League expressed “deep dissatisfaction for the Syrian regime’s acts of oppression,” particularly the use of heavy weapons against its own people. It urged Syria “to stop the cycle of killing and violence and lift the siege off the Syrian neighborhoods under attack.”

The group’s deputy chief, Ahmed Ben Hali added that the Arab states were preparing a resolution in front of the United Nations General Assembly calling for the creation of safe havens to protect civilians and to apply further sanctions on the regime.

Measures passed in the General Assembly are largely symbolic and not binding. The West and its Arab allies have been unable to pass effective resolutions in the more powerful Security Council. China, and especially Syria’s close ally, Russia, have vetoed any attempt to sanction Bashar Assad’s regime.

Russian Foreign Minister Sergey Lavrov on Saturday called the bloodshed in Aleppo a tragedy, but asked what else could the government do against the rebellion.

“Now the city of Aleppo is occupied by the armed opposition; another tragedy is imminent there,” he said. “How can it be hoped that in such a situation the government will simply give in, say ‘Okay, I wasn’t right, overthrow me, change the regime — it’s simply unrealistic.”

Russia has been a key source of support for Syria, although Moscow officials in recent months have said they are simply taking a more even-handed approach while the West offers support to the rebels.

French President Francois Hollande even chided Russia and China on Saturday, asking them to “take into consideration … that it will be chaos and civil war if at some moment Assad isn’t stopped.”

It’s been a difficult two weeks for the Syrian government, with assaults on its two main cities, a bomb that killed four top security officials and a string of high-profile defections.

The country’s military apparatus, though, has remained intact and continues to crush the opposition’s remnants in Damascus and its outskirts.

If they really try to make a stand in Aleppo, the rebels risk being annihilated by superior firepower, and may instead withdraw to preserve their forces as they did in Damascus last week.

Italy welcomed Friday’s release of two Italian electrical engineers, who had been captured eight days ago by militants.

Domenico Tedeschi and Oriano Cantani, who worked on power plants, told reporters in Damascus that they had been kidnapped by five or six masked men who intercepted their car as they drove to the airport. They were later rescued by the Syrian army.

Amid the fighting, three Syrian athletes took part in the first day of competition in London’s Olympic games in swimming, shooting and boxing.

All were defeated, including Wessam Slama, a bantam weight boxer who was one of Syria’s better medal hopes.

The team’s leader, Maher Khayata, whose family is currently trapped by the fighting in Aleppo, said their thoughts were always on the situation at home.

“We would like to return with an Olympic medal,” he told The Associated Press, “but what we want more is to return to our homeland with the news that fighting has stopped and nobody is being killed anymore.”

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Associated Press writers Albert Aji in Damascus, Colleen Barry in Rome, Barbara Surk in London and Jim Heintz in Moscow contributed to this report.

Copyright 2012 The Associated Press.