Air strikes have bombarded rebel-held eastern Aleppo for the first time in two weeks, signalling the start of a new government offensive in Syria's northern city.
Syrian state television said Syria's air force carried out attacks against what it called "terrorist strongholds" and supply depots in the old city of Aleppo on Tuesday, citing what it described as "special sources".
Russian Defence Minister Sergei Shoigu said on Tuesday that fighter jets from the Admiral Kuznetsov aircraft carrier deployed in the eastern Mediterranean also launched their first strikes on Syria.
"For the first time in our naval history, the aircraft carrier Admiral Kuznetsov started taking part in combat," Shoigu said. "Today from this carrier, our Sukhoi-33 began their operations."
He said Russian warplanes would target ammunition depots, training camps and armaments factories in the rebel-held province of Idlib and the central province of Homs.
"We launched a major operation to strike the positions of the Islamic State and Jabhat al-Nusra in the provinces of Idlib and Homs," he said. 
Syrian activists reported strikes in Idlib, Homs and Aleppo.
The UK-based Syrian Observatory for Human Rights said missiles fired from Russian warships in the Mediterranean struck areas in Aleppo province and on the eastern edge of nearby Idlib.
The Observatory and other groups said air strikes also struck at least eight neighbourhoods in Aleppo city, killing at least 10 people.
In the northwestern province of Idlib, a member of the Jund al-Aqsa group known as Abu-Dardaa-al-Shami told the AP news agency over the phone that the air strikes were "much, much, much more intense than previous ones". He added that in addition to the air raids, ballistic missiles hit the province near the town of Saraqeb.
Later on Tuesday, the Russian Interfax news agency cited Russia's defence ministry as saying that Tuesday's missile strikes did not hit Aleppo. The ministry said that Russian and Syrian jets have not bombed the city in the last 28 days.
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Aleppo has become the fiercest front in Syria's five-and-a-half-year war, pitting President Bashar al-Assad, supported by Russia, Iran and Shia militias, against Sunni rebels, including groups backed by Turkey, the United States and Gulf monarchies.
An Aleppo-based spokesman for the Syrian Civil Defence group said air raids hit the Haidariya, Hanano and Sakhour neighbourhoods.
"It's all air strikes and parachute bombs. Today, the bombing is violent ... There hasn't been this kind of attack in more than 15 days," said Ibrahim Abu al-Laith, adding fighter jets and helicopters were still visible in the sky.
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The Syrian Civil Defence, also known as the White Helmets, is a first responder group that operates in rebel-held areas across the country. 
The Syrian Observatory for Human Rights said three people had been killed in rebel-held Aleppo on Tuesday, and strikes also hit the Sheikh Faris, Bab al-Nairab, Qadi Askar and al-Qaterji districts.
On Monday, air strikes hit two hospitals in rebel-held areas west of Aleppo, wounding a number of patients and medics, according to rescue workers and a doctor.
Western countries and human rights activists have accused the air forces of the Damascus government and its Russian ally of repeatedly targeting hospitals, bread lines and other civilian infrastructure in territory controlled by the rebels.
Both Moscow and Damascus have denied doing so, and say their air campaign is directed against military targets belonging to the rebels, who they describe as "terrorists".
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Aleppo has for years been divided into government-held western and rebel-held eastern sectors, but the Syrian army and its allies managed to isolate and besiege the rebel-held districts during the summer. Its allies include Iran's Revolutionary Guards, Lebanon's Hezbollah and Iraqi Shia militias.
A government offensive to retake eastern districts raged from late September to late October, backed by an intense aerial bombardment that the United Nations said killed hundreds and drew condemnation from western countries and rights groups.
However, after Damascus and Moscow announced their pause in strikes last month - a move they said was to let those who wished to quit besieged areas - rebels launched their own assault that killed dozens of civilians in west Aleppo, according to the UN.