Russia has launched new strikes in Syria involving carrier-borne jets and long-range missiles as opposition activists on the ground reported that the first air strikes in nearly a month had struck rebel-held parts of the northern city of Aleppo.
According to activists, between four and 10 people were killed in the strikes on Aleppo, raising concerns of a return to daily deadly bombardments in Syria's largest city.
The Russian blitz began hours after President Vladimir Putin and US President-elect Donald Trump discussed Syria over the phone and agreed on the need to combine efforts in the fight against what the Kremlin called the number one enemy - "international terrorism and extremism".
The Obama administration has been trying for months to negotiate a ceasefire in Aleppo, a city that has become the epicentre of the war between President Bashar Assad and rebels fighting to topple him, some of whom receive US aid.
From the militants' side, al Qaida's Syrian affiliate is fighting alongside the rebels, but Islamic State has no presence in Aleppo city.
Russian defence minister Sergei Shoigu reported to Mr Putin that Su-33 jets from the aircraft carrier Admiral Kuznetsov are involved in the operation, marking its combat debut.
"We launched a major operation to strike the positions of the Islamic State and Jabhat al-Nusra in the provinces of Idlib and Homs," Mr Shoigu said at a meeting with Mr Putin, referring to the al Qaida branch in Syria by its old name. He said the Russian strikes are targeting ammunition depots, training camps and armaments factories.
As part of the Russian operation, a navy frigate launched long-range Kalibr cruise missiles and the military also used Bastion missiles stationed in Syria to strike militant targets in the rebel-held northern province of Idlib and the central province of Homs.
Mr Shoigu said that before launching the strikes, military intelligence had thoroughly identified and selected targets, including factories producing toxic chemical agents.
"It's a well-developed, industrial-scale production," he said, adding that the strikes will continue. He did not mention Aleppo.
Syrian activists reported strikes in Idlib, Homs and Aleppo.
The Britain-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's chief, Rami Abdurrahman, said the areas hit with missiles are strongholds of jihadi groups, including the al Qaida affiliate known as Fatah al-Sham Front.
The Observatory and other groups said air strikes also hit at least eight neighbourhoods in Aleppo city, killing at least 10 people. The Local Co-ordination Committees, another activist group, said the air strikes on Aleppo killed four and wounded others.
Ibrahim al-Haj, of the Syrian Civil Defence, said that by late afternoon his group had counted 72 air raids in nine neighbourhood of eastern Aleppo. He said four people were killed and 32 were wounded in attacks that mostly hit residential areas in the besieged city.
Russian Defence Ministry spokesman Major General Igor Konashenkov insisted that the Russian military did not strike any targets in the city of Aleppo, adding that Russian and Syrian warplanes have not conducted any raids on the city for four weeks.
Activist Baraa al-Halaby, based in eastern Aleppo, said warplanes were firing missiles and helicopters were dropping barrel bombs on the eastern part of the city.
A reporter for Syrian state TV, speaking from the government-held western part of the city, said troops were preparing to attack eastern Aleppo from nine directions.
Four weeks ago, Russia said it would halt weeks of air strikes to allow rebels and supporters to leave eastern Aleppo. The rebels refused to take up the offer and the United Nations failed to negotiate the delivery of aid into the besieged area.
On Saturday, Syrian government troops repelled a rebel offensive on the western parts of Aleppo designed to break the siege on the opposition-held east. The government siege has left about 275,000 people trapped, with no aid allowed in since July, amid a punishing bombing campaign.
In the lead-up to the offensive, a number of air strikes targeted at least three field hospitals in rural western Aleppo province, putting them out of order, the Observatory and Thiqa news agency reported.
Syrian and Russian governments deny attacking hospitals. The UN rights agency, Unicef and other UN agencies have repeatedly condemned air strikes on hospitals and medical facilities.
In Geneva, meanwhile, two UN agencies said food production in Syria has shrunk to "an all-time low", threatening to drive more people from their homes after more than five years of war.
The World Food Programme and the Food and Agricultural Organisation said factors like rising prices, poor weather, general instability and the lack of fertiliser and seeds could force some farmers to stop producing.
A joint report said the planting area in the 2015-16 season was the smallest ever in zones controlled by the government, ethnic Kurds and the armed opposition.
WFP spokeswoman Bettina Luescher said information was lacking about areas controlled by IS. FAO regional representative Abdessalam Ould Ahmed said 80% of households in Syria lack food or money to buy it.