Syrian government forces mounted heavy air strikes against rebel positions in and around the northern city of Aleppo on Friday, the day after a major Islamist-led offensive on areas of the city controlled by President Bashar al-Assad.
Thursday's rebel attack, the most intense insurgent offensive in Aleppo in three years, aimed to build on recent advances against Assad by an array of groups including Islamic State and rebels backed by his regional foes.
Aleppo, 50 km south of the Turkish border, was Syria's most populous city before the country's descent into civil war. It has been partitioned into areas of government and insurgent control since 2012.
Losing Aleppo would further restrict Assad's sway to western areas of the country near the border with Lebanon, where he has sought to shore up his control with the help of the Iranian-backed Lebanese group Hezbollah.
Fighting between the insurgents and government forces continued into the early hours of Friday, and Syrian army air strikes on rebel positions were continuous, the Syrian Observatory for Human Rights, a group monitoring the war, said.
A Syrian military source said the attack had been repelled and heavy casualties had been inflicted on the insurgents, adding that the air force and artillery had been used to target the rebels, who he said had used heavy weapons in their attack.
The Observatory's Rami Abdulrahman said the rebel forces had seized some buildings from government control on the north-western city outskirts, but the advance was not of strategic importance. He said the town of Azaz in the countryside north of Aleppo had also been targeted in the air strikes.
An insurgent alliance including the al Qaeda-linked Nusra Front and the hardline Islamist Ahrar al-Sham said they had set up a joint operations room to run the offensive to "liberate" Aleppo and later govern it according to Islamic sharia law.
Security sources in Turkey, one of the countries most hostile to Mr. Assad, said the Turkish authorities had deployed additional troops and equipment along part of its border with Syria as fighting north of the city of Aleppo intensified.
But Prime Minister Ahmet Davutoglu said there were no immediate plans for any incursion.
The Syrian government has said Turkish assistance to the rebels has been crucial to their advances in the north-western province of Idlib, most of which has fallen to the insurgents since they captured its provincial capital in late March.
The military source said the rebels had bombarded government-held parts of Aleppo with weapons including highly destructive "hell canons" - improvised mortar bombs made out of cooking gas cylinders.
In addition to most of Idlib province, Mr. Assad has also recently lost the central city of Palmyra to Islamic State, and areas of southern Syria to an alliance of rebels known as the "Southern Front" that profess a moderate vision for Syria.
In an apparent effort to stem the losses, the army has put up stiff resistance to an Islamic State attempt to seize government-held areas of the north-eastern city of Hasaka, and has also been fighting hard against a rebel push to captured the southern city of Deraa.
