The Associated Press reports: Syrian rebels have launched an offensive to “liberate” the country’s largest city of Aleppo, an opposition commander said Sunday, while in Damascus government troops backed by helicopter gunships wrested back control of rebel-held neighborhoods.

The attack on Aleppo, Syria’s commercial hub and traditionally a bedrock of support for President Bashar Assad, was a sign of the rebels’ growing confidence and capabilities even as regime forces appeared close to regaining control of the capital Damascus after days of intense street battles there.

With Syria’s civil war moving from the countryside and smaller cities into the country’s two main urban centers, an activist group said the death toll had risen to more than 19,000 since the uprising began in March 2011. The Syrian Observatory for Human Rights also said July is shaping up to be the deadliest month of the conflict so far, with 2,752 people killed in the first three weeks.

The bloodshed has escalated as the rebels have taken the fight to the government with a week of fighting in Damascus, including a bombing that struck at Assad’s inner circle and killed four senior regime officials. In a bid to seize the momentum, the opposition also has taken control of four border crossings with Iraq and Turkey, most recently the Bab al-Salamah post on the Turkish frontier.

A video posted online by activists Sunday showed about a dozen gunmen standing in front of the Bab al-Salamah crossing as they raised the Syrian opposition flag.

Gen. Qassim al-Dulaimi, commander of Iraq’s forces around the border region of al-Qaim, reported the sounds of fighting at the Bukamal crossing, suggesting Assad’s troops are trying to retake that frontier post.

The fighting in Damascus and Aleppo has shaken the government’s once seemingly iron grip on the two cities, which are both home to elites who have benefited from close ties to Assad’s regime, as well as merchant classes and minority groups who worry their status will suffer if Assad falls.

Col. Abdul-Jabbar Mohammed Aqidi, the commander of rebel forces in Aleppo province, said “we gave the orders for the march into Aleppo with the aim of liberating it.”

“We urge the residents of Aleppo to stay in their homes until the city is liberated,” he said in a video posted by activists on YouTube. He added that rebels were fighting inside the city while others were moving in from the outskirts.