A smothering Syrian blockade and round-the-clock airstrikes have sought to starve and shock rebel-held areas of Aleppo into surrender. On Friday came the counterpunch: The rebels launched an apparent last-ditch effort to end the government’s siege.

A failure by the rebels — a patchwork of militiamen and Islamist factions — to gain some breathing room could be a crippling blow to efforts to hold one of the last opposition strongholds.

Forces of the Syrian government and its partners, including Russia and Iran, have vowed to slam eastern Aleppo until the resistance is broken. A drawn-out fight in Aleppo, however, would display the resolve of opposition groups and force President Bashar al-Assad to decide whether to fight on or make an embarrassing pullback.

The gambit by the rebel forces seeks to take the battle to the government-held lines. The immediate aim is to open routes for supplies and reinforcements into rebel zones in eastern Aleppo, where more than 200,000 people face death in air raids and as a result of severe shortages of food and medicine.

Ahead of Friday’s assault, rebel fighters released videos declaring that their offensive would deal a blow to Assad and his allies, including Shiite militiamen from Iran, Lebanon and Iraq. Russia backs Assad with warplanes and other battlefield equipment.

The rebels fired artillery indiscriminately at government-held neighborhoods in Aleppo, bringing a surge in violence to relatively calm areas. The Britain-based Syrian Observatory for Human Rights, which monitors the conflict, said the shelling killed 15 people and wounded more than 100 in western Aleppo.

Jabhat Fatah al-Sham — an al-Qaeda-linked, anti-Assad force formerly known as Jabhat al-Nusra — said it carried out at least one suicide bombing. The various claims of casualties and attacks could not be independently verified.

“Bad news for Bashar and his gangs. Our fighters love to die in the same way Bashar loves to live,” a fighter from the Islamist Faylaq al-Sham unit says in a video, referring to the Syrian leader. The pronouncement is made against a background of Koranic verses.

The battle for Aleppo has become a crucial struggle in the more than five-year-old Syrian conflict because of its symbolic and strategic significance. It was Syria’s largest city before the war, with a population then approaching 3 million. Many residents have since fled.

Rebels stormed Aleppo’s eastern districts in 2012, and the area is now their last major urban base.

The city also has increasingly taken on international significance amid heightened friction between the West and Russia, which intervened militarily on Assad’s behalf last year.

Moscow’s aircraft have joined government warplanes in intensifying attacks on hospitals and residences in eastern Aleppo in recent weeks. Moscow has denied links to some airstrikes, including an air assault on a school in northern Syria this week that left more than two dozen people dead, many of them children.

Rising violence by Syria and its allies scuttled a cease-fire brokered last month by the United States and Russia. The bloodshed has drawn an international outcry because of widely circulated images from Aleppo showing devastation such as bombed-out hospitals and dead civilians, including children.

In Moscow on Friday, Syria’s foreign minister, Walid al-Moualem, said the government could agree to a “humanitarian pause” in attacks on Aleppo if rebel forces allow civilians to leave areas they control.

But the foreign ministers of Russia and Iran told their Syrian counterpart that they would intensify the fight against “terrorism” in Syria, without giving specific details on potential targets.

The presence of al-Qaeda-linked fighters and other Islamist militants in battles for the city has introduced major political complications to the conflict. Syria and Russia claim they are seeking to fight “terrorists,” a term that they regularly use to describe most, if not all, of Assad’s opposition. And the West and its partners have been unable to draw clear distinctions between the militias they back and the extremist forces.

Relatively moderate rebels play down the role of the extremists, even as al-Qaeda-linked fighters and allied Islamist militants exert considerable sway over the rebellion, which began in 2011 with peaceful rallies during the Arab Spring uprisings.

Western officials have expressed concern about a flotilla of Russian warships — including an aircraft carrier capable of holding 40 planes — that appears to be making its way toward the Syrian coast, possibly to help Assad’s forces in Aleppo.

Lt. Col. Abu Bakr, commander of the Jaish al-Mujahideen rebel group, said the new offensive has yielded success.

“We’ve launched tens of rockets on regime positions, and we’ve broken through the government’s first line of defense,” said Bakr, speaking from Aleppo over the WhatsApp messenger service. “There are about 5,000 rebels participating, and we think this will last a week” until the siege is broken.

Syrian state media, however, reported that government forces repelled the rebel assaults.

Zakaria Zakaria in Istanbul contributed to this report.