Russia’s Admiral Kuznetsov carrier is pictured in the Barents Sea in 2014. (Associated Press)

With a hail of missile strikes, Russian forces announced a major offensive Tuesday against rebel-held areas in Syria. As the assault unfolded, activists in the crucial city of Aleppo said that their skies were filled with government warplanes and that bombs were raining down.

The Russian Defense Ministry said that it had used its sole aircraft carrier — ordered last week to the Mediterranean Sea in support of Syrian President Bashar al-Assad — for the first time and that missiles had also been fired from a land-based system inside Syria as well as another Russian warship off the coast.

Activists said the Russian targets appeared to include eastern Aleppo, one of the last urban bastions of factions opposing Assad. But Moscow denied involvement in the attacks on the city, saying it has been nearly a month since Russian forces launched strikes there.

The divided city has become the epicenter of the battle for Syria. The government’s recapture of Aleppo could hasten the fall of remaining rebel strongholds across the country.

Russia’s assault kicked off just hours after Putin and President-elect Donald Trump, speaking by phone, agreed to combine efforts in Syria to defeat what Moscow has said is its enemy in the fight: “international terrorism and extremism.”

The Obama administration has been aiding what it sees as moderate opponents of Assad while continuing the fight against al-Qaeda’s Syrian affiliate, which in some places has been battling alongside the rebels.

Russia describes all rebel groups as “terrorists,” and Trump’s blanket statements about joining Russia have been seen in Moscow as tacitly supporting this view.

Russian Defense Minister Sergei Shoigu said “massive strikes” were waged against positions of the Islamic State and an al-Qaeda-linked militant group in Idlib and Homs provinces. The regions are north of Damascus and include rebel-held zones but do not cover Aleppo.

Shoigu told Putin in a meeting in the Russian city of Sochi that Su-33 jets were launched from the aircraft carrier Admiral Kuznetsov and that the frigate Admiral Grigorivch launched cruise missile strikes.

Shoigu said the attacks targeted industrial sites used by rebel forces to make toxic substances used as a “serious means of mass destruction.” The Russian claims could not be independently verified.

In August, a team of international inspectors determined that the Syrian government and Islamic State militants were responsible for chemical attacks carried out in 2014 and 2015. Syria denied the findings and, in turn, has claimed that rebel forces and other militants have waged chemical attacks.

The United States condemned the renewed attacks and said they had struck civilian targets. “The most recent reported attacks are on five hospitals and one mobile clinic in Syria. We believe it’s a violation of international law,” State Department spokeswoman Elizabeth Trudeau said Tuesday in Washington.

In Aleppo, civilians and rebel fighters said missiles and barrel bombs struck targets across at least five neighborhoods in the city’s rebel-held east, an area controlled by anti-Assad fighters. The White Helmets civilian rescue group said it documented more than 100 attacks, some involving cluster munitions.

The military balance in Aleppo has become a bellwether for the state of Syria’s war.

When rebel forces captured the city’s eastern districts in 2012, they appeared ascendant, boasting that a march on Damascus would be next. Four years later, the area they control lies in ruins, and opposition fighters have been outgunned by Russian, Syrian and Iranian firepower.

Images shared on social media from east Aleppo on Tuesday showed men and children searching for survivors in the wreckage of a shattered building. The casualty count was not immediately clear.

Syrian state television said the strikes hit what it called “terrorist” strongholds and supply depots. But residents said many of the bombs struck homes. The claims also could not be independently verified.

In the city of Homs, the assault appeared to have involved incendiary weapons. Photographs from al-Waer — an area of the city where the Syrian government is trying to force rebels to surrender — seemed to show several charred bodies.

Russia has promised an all-out offensive to retake Aleppo, giving fighters and civilians a sunset deadline earlier this month to leave rebel-held districts. For 15 days, the city had waited, watching as warplanes streaked overhead, only to drop their loads on the western countryside instead.

But by midday Tuesday, the attacks resumed.

“You could hear airstrikes all over the city,” said Amir Ragab, 54. “We’ve had so many false rumors and promises about this final assault — I can only live day by day and put my faith in God.”

Some spoke of terror, others of exhaustion. Sending photographs of smoke billowing above Aleppo’s skyline, the head of the local White Helmets branch, Ammar al-
Selmo, said his workers were unable to reach many of the wounded while the attacks continued.

“Maybe it is better with bombing,” he said. “Then at least you die quickly.”

Filipov reported from Moscow. Heba Habib in Stockholm contributed to this report.