Heavy fighting continued in Aleppo on Sunday, with army tanks shelling Syria's second city and a helicopter gunship strafing rebel positions, according to news reports.
Meanwhile, it emerged that Syria's first astronaut, General Mohammad Faris, may have become the latest senior officer to defect from the Syrian army.
The BBC cited military sources as saying that around 20,000 troops were massed around the city, and CNN reported that fears were growing of a full-scale assault on rebel elements there within days.
Free Syrian Army fighters told CNN that two large columns of government troops were heading toward Aleppo — a key battleground of the 17-month uprising — from Latakia on the Mediterranean coast and the from Damascus.
The reports come after days of heavy fighting in the city — worsened by the resignation of peace envoy Kofi Annan and disintegration of his cease-fire plan.
United Nations Secretary-General Ban Ki-moon called Aleppo "the epicenter of a vicious battle between the Syrian government and those who wish to replace it" and said he feared the violence could escalate even further, CNN reported.
On Friday, the UN General Assembly passed a non-binding resolution condemning the Syrian government and placing blame on their own Security Council's failure to take strong, united action in Syria, according to Reuters.
Russia said the UN vote was a "facade of humanitarian rhetoric" and that international enemies of Assad were arming the rebels and escalating the fighting, Reuters reported.
A government offensive has caused widespread destruction and forced tens of thousands of civilians to flee, CNN wrote.
Reuters cited state television as saying that Assad's forces were "cleansing the terrorist filth" from the country, with tanks pounding alleyways in pursuit of rebel fighters.
A day earlier, military jets pounded Damascus, a continuation of fighting that flared in the capital shortly before a July 18 bomb killed four of Assad's inner circle, Reuters wrote.
Faris, the first Syrian visitor to the Mir space station according to SpaceToday.org, fled to Turkey on Sunday, the Anatolia news agency reported.
According to YNet News, the Syrian opposition posted a video in which Faris confirmed he had joined the rebels and was fighting Assad's forces in northern Syria.
Faris carried along a portrait of Assad's father and predecessor, President Hafez al-Assad, and held a video conversation with him when he flew in space in 1987.
Separately, 48 Iranian pilgrims were kidnapped by gunmen near a Shia shrine in Damascus on Saturday with Iranian diplomats and Syrian state television blaming the abduction on "armed groups."
Fox News reported Sunday that a pan-Arab television station had aired a video purporting to show Syrian rebels guarding the Iranians and promising more attacks on Iranian targets.
Tehran has had asked Turkey and Qatar to help win the Iranians' release, the BBC wrote.
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