BEIRUT — Islamist rebels breached neighborhoods in Syria’s second-largest city of Aleppo and clashed with government military forces after detonating two car bombs Friday. The incident, reported by a leading war monitoring group and The Associated Press, has renewed international attention on a country wracked by civil war and extremism for more than a decade.

The Syrian Observatory of Human Rights, the monitoring organization, said fierce clashes were underway between the attacking insurgents and regime troops. The fighting is a significant escalation since rebels launched a surprise offensive Wednesday, seizing towns and villages as they advanced toward Aleppo.

The breach marked the first time opposition forces besieged the city since 2016, when they were driven out of Aleppo’s eastern neighborhoods during a military operation in which Syrian troops were supported by Russia and Iran. Four years ago, a ceasefire brought an end to the most intense violence, but the new push from rebels has upended a period of relative calm.

The rebels appear to have gained momentum by the diminished strength of Iran-backed groups such as Hezbollah throughout the region, illustrating how the conflicts in the Middle East feed off one another.

Witnesses who spoke to The Associated Press said Aleppo residents were fleeing from areas on the western edge amid missiles and exchanges of gunfire. The AP said an insurgent commander posted a recorded message on social media urging the city’s residents to cooperate with the rebels. The rebels are led by the militant group Hayat Tahrir al-Sham, or HTS.

Syrian armed forces were repelling attacks, the Syrian Defense ministry said Friday, adding government forces involved in combat operations were able to regain control over some areas.

The Syrian Observatory for Human Rights, which relies on a network of on-the-ground information sources, said at least 121 people have been killed since the start of HTS’ shock offensive, including at least 20 civilians.

Syrian state media reports cited by the AP said insurgent-fired projectiles struck student accommodations at Aleppo’s university in the city center, killing four people, including two students.

The Syrian civil war started in early 2011 with a wave of protests against the authoritarian rule of President Bashar al-Assad. Assad’s government, bolstered by arms shipments from Iran and Russia, cracked down forcefully on the popular uprisings, setting off a spiral of violence and fueling an aggressive insurgency.

Matt Bradley reported from Beirut and Daniel Arkin from Atlanta.