Syrian rebels in the northern city of Aleppo Friday said they were bracing for the "mother of all battles," as government troops and rebels deployed reinforcements to the area.

"We are ready for the mother of all battles," Abu Omar al-Halabi, a commander in the Free Syrian Army based near the south-eastern district of Salaheddine in Aleppo, told dpa by phone.

The two sides have have been fighting for control of Syria's commercial hub since the weekend.

Al-Halabi said more than 3,000 rebel fighters from across Syria had joined the 2,500 already positioned in Aleppo since Thursday.

The government was also sending reinforcements to the city, where its helicopter gunships and artillery were bombarding rebel-held areas, opposition activists said.

The troops were being deployed to the area from Idlib, near the border with Turkey.

"The clashes with the regime forces did not stop throughout the night. Some roads leading to Aleppo's airport are now under the rebels' control," al-Halabi said.

The United States said on Thursday it appeared that forces loyal to Syrian President Bashar Assad were "lining up" for a massacre in the city of Aleppo, but again ruled out military intervention in the conflict.

The U.S. State Department said that credible reports of tank columns moving on Aleppo along with air strikes by helicopters and fixed-wing aircraft represented a serious escalation of the government's efforts to crush an armed rebellion.

In the Syrian capital Damascus fierce clashes between troops and rebels entered a second day in the southern al-Hajar al-Aswad area, one of the last districts controlled by the opposition in the capital.

Forces loyal to President Bashar Assad this week recaptured several districts of Damascus that had fallen to the rebels.

At least 200 people were killed Thursday across Syria - 48 of them in Damascus and 31 in Aleppo, the opposition Syrian Observatory for Human Rights said.

Syrian state media, citing a security source, reported that troops were fighting "terrorists" in Aleppo, a city of around 2 million people.