BEIRUT — Syrian attack helicopters and warplanes circled overhead in the country’s largest city of Aleppo, as fierce fighting continued in the former regime stronghold for a fourth day, and rebel fighters struggled to hold on to ground they gained earlier this week.
In Washington, Secretary of State Hillary Rodham Clinton sent “a clear message of support for the opposition,” while Syrian ally Russia warned Syrian President Bashar Assad against using chemical or biological weapons against a foreign attack.
“We are well aware that the pace of events is accelerating inside Syria,” Mrs. Clinton said. “We have to work closely with the opposition … .”
The Russian Foreign Ministry reminded Mr. Assad that Syria has signed a convention banning chemical weapons and that he must “unfailingly honor international obligations.” Mr. Assad’s government on Monday confirmed it possessed weapons of mass destruction and threatened to use them against foreign invaders.
Iran, another top Syrian ally, pledged to land “decisive blows” to prevent the overthrow of Mr. Assad.
“The Syrian people and the friends of Syria will not allow regime change,” said Brig. Gen. Masoud Jazayeri, the deputy chief of Iran’s joint armed forces.
In Damascus, Mr. Assad named Gen. Ali Mamluk as his new national security chief in a shakeup of intelligence services, following a bombing that killed four top regime officials last week.
Aleppo, a commercial hub and home to 2.5 million people, has become a new front in the 16-month uprising against the Syrian regime.
“The city is a real war zone now with artillery shelling parts of the city, as well as tanks and helicopters assaulting neighborhoods,” said an activist in the city who goes by the pseudonym Edward Dark. “People have fled their houses with nowhere to go. “
Government forces also violently suppressed a prison riot in Aleppo early Tuesday morning with activists reporting that at least eight people had been killed as parts of the prison were set on fire.
Residents in areas close to Aleppo’s historic old city reported the sound of gunfire, suggesting battles between government troops and rebels of the Free Syrian Army had reached entrances to the UNESCO world heritage site.
“The regime has been saying all along that it is still strong because it controls Aleppo and [the capital of] Damascus,” said Nadim Shehadi of the London-based think tank Chatham House. “The regime kept very tight control over Aleppo and Damascus and used them as an argument that to a certain extent it worked.”
Mr. Shehadi added that the fighting in Aleppo and Damascus has been extremely damaging to the regime’s claims that it is in control of the country.
Rebels have been battling to hold areas they have taken west and north of Aleppo, including a border crossing at Baba al-Hawa.
The Free Syrian Army has so far failed to occupy any urban area in Syria and has resorted to guerrilla tactics.
Meanwhile, tensions have flared in recent days between the Free Syrian Army and Kurdish fighters in the northeast of Syria, who had ordered the rebel army to stay out of Kurdish towns.
Kurds have suffered more than most under the Assad regime, with high rates of poverty and their culture and traditions suppressed. Many have been denied Syrian citizenship. They remain divided over opposition to the regime, with many fearing what would replace it.
* Louise Osborne in Berlin contributed to this article, which is based in part on wire service reports.
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