ALEPPO, Syria — Rebels had seized most of Syria’s largest city, Aleppo, as of Saturday, according to a war monitoring group and fighters combing the streets in search of any remaining pockets of government forces.

The antigovernment rebels said they had faced little resistance on the ground in Aleppo. But Syrian government warplanes responded with airstrikes on the city for the first time since 2016, according to the monitoring group, the Syrian Observatory for Human Rights.

Aleppo came to a near standstill Saturday, with many residents staying indoors for fear of what the sudden flip in control might mean, witnesses said. Some rebels tried to reassure residents and sent out at least one van to distribute bread.

The rapid advance on Aleppo came just days into a surprise rebel offensive launched Wednesday against the autocratic regime of President Bashar al-Assad. The developments are the most intense escalation in years in a civil war that had been mostly dormant.

The timing of the assault suggested that the rebels could be exploiting weaknesses across an alliance linking Iran to the militant group Hezbollah in Lebanon as well as the Assad regime in Syria and others.

In Aleppo on Saturday, opposition forces said that although they were in control of nearly the entire city, they had not yet solidified their hold on it. The rebels also announced the capture of towns and cities across two provinces, Aleppo and Idlib.

Within hours from Friday into Saturday, Syrian government soldiers, security forces and police officers fled the city, according to the monitoring group.

The Russian Defense Ministry said Friday that Moscow had carried out airstrikes against the rebel offensive in support of the Syrian military. The monitoring group said that Russian strikes had hit opposition-held areas of Aleppo and Idlib provinces.

The Syrian state news media challenged the reports of a rebel takeover of most of Aleppo.

The rebel alliance is led by the group Hayat Tahrir al-Sham, which was once linked with al-Qaida. Turkish-backed rebel groups have also joined in.

Kurdish-led fighters — a separate force backed by the United States that is not aligned with the rebels — also took over abandoned checkpoints in some Aleppo neighborhoods, according to the monitoring group and rebels.

Some city residents said they were concerned about what a takeover by a hodgepodge of rebel groups could bring. But that is not their only fear.

The airstrikes Saturday could portend more aerial attacks and a return to the years when rebels controlled parts of the city that were bombarded regularly by regime and Russian warplanes.

In December 2016, the Assad government, aided by Russia, recaptured all of the city after a prolonged siege of opposition-held neighborhoods. The takeover was exalted at the time by Assad as a turning point in the war.

In the years that followed, his regime, supported by the Russian military and Iran-backed militias, clawed back large parts of the country.