BEIRUT — Syrian army helicopters fired on neighborhoods in Aleppo on Friday morning, activists said, as the army readied assault troops and armored columns for a possible invasion of the city, a densely populated commercial capital where insurgents have embedded themselves over the past week in preparation for a battle.

As Aleppo girded for fighting, the top U.N. human rights official expressed alarm over the possibility of heavy bloodshed in the city. The official, Navi Pillay, the High Commissioner for Human Rights in Geneva, who has been highly critical of the Syrian government, said she was also deeply concerned over reports of heavy bombardments, atrocities and extrajudicial killings committed during fighting in Damascus and other urban combat zones.

“All this, taken with the reported buildup of forces in and around Aleppo, bodes ill for the people of that city,” she said.

In a further sign of unraveling in Syria, the International Committee of the Red Cross, one of the few aid agencies working in Syria, said it was temporarily evacuating some staff from Damascus to Beirut because of the rising security dangers. Philip Stoll, a Red Cross spokesman in Geneva, also said several schools in Aleppo had been opened to shelter residents fleeing their homes in search of safety.

Fears of an impending Aleppo battle came as Syrian opposition figures said that a member of the Syrian Parliament from the city’s northern district had defected and crossed into Turkey. The lawmaker, Iklhas Badawi, was elected in May to a Parliament that was seen as a rubber stamp for the predominance of President Bashar Assad’s Baath Party. The elections were dismissed by opposition figures as a sham. A local official in Turkey who confirmed that Badawi crossed the border said she would not be placed in one of the refugee camps on the border because of her “special status.”

The government’s opponents, anxious for news of cracks in Assad’s government, have cheered the defections of even minor functionaries. “We will work on helping her and making her feel at ease,” said George Sabra, a member of the largest Syrian opposition group. “The regime is isolating patriots.”

The Syrian military shelled rebel targets in urban enclaves on Thursday. Antigovernment activists reached by phone and Skype in Aleppo said that the city’s civilian population was gripped by foreboding as government forces massed on the southern outskirts, and that fierce street clashes had sporadically erupted. But Syrian military commanders appeared to be awaiting reinforcements before issuing invasion orders.

Military experts have long speculated that Assad’s army, which has been scrambling to crush rebel resistance in urban areas like Homs, Hama and more recently central and southern neighborhoods of Damascus during the uprising, lacked the military resources to take on an armed rebellion in all major cities at once. That seemed to explain the delay in Aleppo, where anticipation of an attack has been building for days.

The United States also expressed alarm about the possibility of mass civilian casualties in Aleppo, a UNESCO World Heritage site and one of the Middle East’s most storied cities. Victoria Nuland, a State Department spokeswoman, told reporters in Washington that there was “concern that we will see a massacre in Aleppo, and that’s what the regime appears to be lining up for.”

But Nuland also indicated that the United States was not reconsidering its stance against military intervention, saying, “We do not think pouring more fuel onto the fire is going to save lives.”