• U.S. official: Syrian regime seems to be readying for massacre

    With the Assad regime directing the full force of its military at Aleppo, Syria's biggest city, the Syrian government is pulling forces out of surrounding towns -- a cause for celebration among rebels there. NBC's Richard Engel reports from inside one of those towns, in northern Syria.

    By NBC News and wire reports

    Updated at 4 p.m. ET:

    The United States has "grave concerns" about the situation in Syria, the State Department said Thursday, as President Bashar Assad's forces withdrew from many towns and villages and focused their firepower on the key city of Aleppo.

    "This is the concern, that we will see a massacre in Aleppo, and that's what the regime appears to be lining up for," said spokeswoman Victoria Nuland.

    Citing "columns of tanks" outside the city, Nuland said the Syrian military "seem(s) to be massing for an attack." She called the use of fixed-wing aircraft in addition to helicopter gunships a "serious escalation in this conflict."

    "It could be a humanitarian disaster for the people of Aleppo," NBC News' Richard Engel reported from northern Syria. "It also means the Syrian troops are forced to make trade-offs. They don't have enough loyal troops to make the offensive against Aleppo and hold these rural areas."

    The key city of Aleppo has come under ferocious assault, bombarded by fighter jets and machine gun fire. The Syrian government's main priority is taking control of the major cities – without enough troops to control the entire country, they are on the offensive. NBC's Richard Engel reports from northern Syria.

    Nuland said Assad's regime was "increasingly losing control of its territory, that there are swaths of the country that are no longer under the control of the regime, that his tactics are increasingly violent, increasingly desperate as that happens."

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    Military experts believe an overstretched Syrian army is pulling back to concentrate on fighting insurgents in Aleppo and Damascus, important power centers for the government, while leaving outlying areas in the hands of rebels. Assad's forces have launched massive counterattacks in both cities.

    Pierre Torres / AFP - Getty Images

    Free Syria Army opposition fighters guard a group of police officers Wednesday after overrunning the Shaar district police post in Aleppo.

    Meanwhile, opposition activists said thousands of troops had withdrawn with their tanks and armored vehicles from Idlib province near the Turkish border and were heading toward Aleppo. Rebels attacked the rear of the troops withdrawing from the north, activist Abdelrahman Bakran told Reuters from the area.

    Fierce clashes raged in the early hours in Aleppo itself, and an activist said rebels now controlled half of the city, a claim that could not be independently verified. 

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    "There was shelling this morning on the Salaheddine and Mashhad districts," Aleppo activist Abu Hisham told Reuters. "Now it stopped, but helicopters are buzzing overhead." 

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    Activists said 24 people were killed in fighting in and around Aleppo on Wednesday, swelling a national death toll of about 18,000 since the revolt against Assad began 16 months ago. 

    'Terrorists are suffering terrible losses'
    State-run Syrian television painted a more favorable picture, saying government troops were imposing security and stability in and around Aleppo.

    "The terrorists are suffering terrible losses. Groups of them are throwing their weapons away and giving themselves up. Others are fleeing for the Turkish border," the television report said. 

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    Engel and other journalists have repeatedly observed government troops retreating from "secondary towns" so they can fight opposition forces in the capital Damascus and now Aleppo, leaving vast swathes of the countryside under rebel control. 

    "When government forces pull out of a place they lose control and immediately rebel flags go up and rebels hold celebrations," Engel said. "Those celebrations have been attacked so now the rebels are deciding not to hold celebrations because they're noisy, they have lights and they're easy to target."

    Total war: Syria sends armored column to Aleppo

    Meanwhile, north of Aleppo, the town of Azaz has been almost completely destroyed by heavy fighting, Reuters reported. Burned-out armored personnel carriers sat on the roads where rebels hit them with rocket-propelled grenades. Bullet casings were scattered everywhere. 

    The Syrian government's army is descending on the northern city of Aleppo after the city was seized by rebels. NBC's John Ray reports.

    In another key development, neighbor Turkey was not allowing goods or people to pass over the Bab al-Hawa border crossing after rebels took base near Aleppo over the weekend, Engel said. 

    "Maybe they're concerned about there being too many rebels or refugees crossing," he said. "There may also be concern that the Turks may be losing control of security in sourthern Turkey."

    Dozens are reported dead in Syria where opposition forces are fighting to maintain control of Syria's commercial capital Aleppo. NBC's Richard Engel reports.

    Fighting in and around Aleppo is expected to prompt an exodus across the Turkish border, where some Syrian refugees are already complaining about poor conditions and have clashed with riot police in disputes over food. 

    "There is not enough food. They have broken our hearts, the Turks. Why are they doing this to us?" a sobbing woman named Umm Omar, with her four children huddled next to her in a camp near the border, told Reuters.

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    Rebels also took the Bab al-Salam border crossing with Turkey over the weekend.

    Artillery and rockets
    Further south, Syrian forces used artillery and fired rockets on Wednesday on the northern Damascus suburb of al-Tel in an attempt to seize it from rebels, forcing hundreds of families to flee, residents and opposition activists said. 

    "Military helicopters are flying now over the town. People were awakened by the sound of explosions and are running away," Rafe Alam, one of the activists, told Reuters by phone from a hill overlooking Tel. "Electricity and telephones have been cut off." 

    The latest massacre began with a military bombardment of the village of Tremsi. After the heavy artillery and shelling, villagers said pro-government militia men swept in to kill at close range. NBC's Richard Engel reports.

    Opposition sources also reported helicopters and machine guns were firing on the neighborhood of Hajar al-Aswad. The slum lies on the southern outskirts of the capital and has been a haven for rebels sneaking into Damascus from the suburbs. 

    NBC News' Kari Huus and F. Brinley Bruton and Reuters contributed to this report.

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