Syrian insurgents carried out two suicide car bomb attacks in an assault on pro-government forces in Aleppo on Saturday.
They also opened a new front northeast of the city in an attempted fightback after territorial gains for President Bashar Al Assad.
Syrian government forces had made a significant advance into the rebel-held northwest this week, seizing the town of Maarat al-Numan, as part of an offensive to secure the main highway between Damascus and Aleppo.
The suicide attacks were carried out by Islamist group Hayat Tahrir Al Sham and targeted the Jamiyat al Zahraa area on the western edge of Aleppo.
A third car bomb was set off by remote control, a source with the group said.
A news outlet linked to the group, Ebaa, published a video which it said showed elite Tahrir Al Sham fighters pledging "allegiance to death and jihad" before the attack on Jamiyat al-Zahraa, watched by the group's leader, Abu Mohammad Al Jolani.
The northwestern corner of Syria, including Idlib province and adjoining areas of Aleppo, is the last major rebel foothold in Syria, where Assad has taken back most of the ground once held by his enemies.
Syrian state news agency SANA said army troops had destroyed four car bombs before they reached their targets in that area.
Syrian army forces were firing rockets and artillery at militant groups on the Jamiyat al-Zahraa front, it said.
Militants had also fired rockets at residential districts of Aleppo.
A military news outlet run by Lebanon's Hezbollah, which fights in support of Assad, said the Syrian army had thwarted a "fierce attack" by the Nusra Front, as Jolani's group was known until it broke ties with al Qaeda in 2016.
The Hayat Tahrir Al Sham source said the attacks had targeted the "Iranian occupation militias", a reference to Iran-backed groups that have fought in support of Assad.
Aleppo city has been under full Syrian government control since 2016, when pro-Damascus forces defeated rebels in the east of the city.
Some 50 km (35 miles) northeast of Aleppo, Turkish-backed Syrian rebels attacked government-held positions near the city of al-Bab, a rebel source and the Syrian Observatory for Human Rights reported. SANA made no mention of an attack in that area.
Turkish forces were not taking part, rebel sources said.
The government's latest offensive in the northwest has triggered a fresh wave of civilian displacements, with hundreds of thousands moving towards the Turkish border.
Turkish President Tayyip Erdogan said on Friday Turkey may launch a military operation in Idlib unless the fighting there is halted.
U.S. special envoy for Syria James Jeffrey said on Thursday the Idlib fighting raised the spectre of an international crisis.
Turkey, which already hosts 3.6 million refugees from Syria, fears a fresh wave of migrants from Idlib. It has 12 military observation posts around Idlib, set up under a 2017 agreement with Russia and Iran, and several of them have since been surrounded by advancing Syrian government forces.