Activist group Syrian Observatory for Human Rights reported this week that more than 2,750 people have been killed in Syria so far this month, bringing the death toll since dissenters began fighting the regime of Syrian President Bashar al-Assad last year to more than 19,000.

The Syrian activist group said that if the current pace of killing continues through the end of July, it will be the deadliest month since the Syrian uprising erupted in March 2011.

Observatory chief Rami Abdul-Rahman said Sunday that 2,752 people — 1,933 civilians, 738 government troops and 81 rebels — were killed in the first 21 days of July.

Abdul-Rahman said June had been the deadliest month so far, with 2,924 deaths.

The average daily death toll in June was 94, while this month it has increased to an average of 131 a day.

Meanwhile, fighting in the country continued this week as Syrian rebels launched an operation to "liberate" the country's largest city of Aleppo from regime forces, an opposition commander said Sunday. At the same time, in the capital Damascus, government troops backed by helicopter gunships wrested back control of rebel-held neighborhoods.

The assault showed that even as Assad's forces looked close to regaining control of Damascus after days of intense street battles there, opposition fighters could still mount a new offensive on Aleppo, Syria's commercial hub and bedrock of support for the regime.

The rebels have put the government on the defensive after a week of fighting in Damascus, including a bombing that struck at Assad's inner circle, killing four senior regime officials. Seizing the momentum, the opposition has also 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 border crossing as they raised the Syrian opposition flag.

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 in a video posted by activists on YouTube that "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, adding that rebels were fighting inside the city while others were moving in from the outskirts.

Aqidi said rebels would respect members of religious and ethnic minorities in the city calling on government troops to defect and join the opposition. He added that rebels would protect members of Assad's Alawite minority sect, an off-shoot of Shiite Islam, saying "our war is not with you but with the Assad family."

The Syrian Observatory for Human Rights and Aleppo-based activist Mohammed Saeed said the fighting was concentrated in several neighborhoods.

Saeed said rebels were in full control of the central Salaheddine district and the nearby Sakhour area. He added that thousands of residents have fled tense quarters of the city for safer neighborhoods and the suburbs.

"Aleppo is witnessing serious street battles," Saeed said adding that many shops were closed. He added that rebels were trying to surround the city's international airport, known as Nairab, in order to prevent the regime from sending reinforcements.

In the capital of Damascus, the Observatory also reported attacks by government forces in the neighborhoods of Mazzeh and Barzeh that had once been held by rebels. It said that troops used helicopters gunships in the attack, causing heavy casualties.

Syrian state TV denied government forces were using helicopter gunships in Damascus, and said the capital was calm and special forces were just mopping up the remnants of the "terrorists" in cooperation with inhabitants. The government refers to those trying to overthrow Assad's regime as "terrorists."

The coming days will be crucial to determining whether the regime can recover from the blows, which have punctured the sense that Assad's hold on the country, or at least its two main cities, was unshakable.

On Sunday, Syrian state news agency SANA said troops had cleansed the once rebel-held Damascus neighborhood of Qaboun. On Saturday, troops captured the Damascus neighborhood of Midan, causing major destruction in the area.

Syria also warned on Sunday that Western and Israeli intelligence officials were trying to take over Syria's radio frequencies and were using them to broadcast fabricated emergency announcements such as the fall of the regime.

Meanwhile, about 100 Israeli Druze students from the northern Golan Heights studying in Damascus sent an urgent request to the Red Cross to enable their return to Israel.

Their request was rejected because the organization said it was impossible to secure their transfer from Damascus through the Quneitra border crossing.