A Russian nurse in a makeshift hospital in Aleppo has been killed by rebel shelling of the Syrian government-held part of the city, officials said.

The death came as the defence ministry in Moscow said a Russian fighter jet crashed into the Mediterranean Sea after returning from a sortie over Syria.

The developments were a blow to Russia, which has been one of the staunchest supporters of Syrian president Bashar Assad in his country's bitter civil war, now in its sixth year.

The shelling that killed the female nurse also wounded two Russian doctors working in the field hospital, a Russian officer in Aleppo said.

The hospital equipment was part of aid that Moscow had sent into the Furqan neighbourhood in the government section of the city the previous day.

In Moscow, defence ministry spokesman Major General Igor Konashenkov confirmed the death of one Russian medic in Aleppo and said two were seriously wounded in an artillery attack that hit the Russian military hospital.

He said an unspecified number of local residents who were at the hospital were also wounded and accused the United States, Britain and France of tipping off the rebels about the hospital's location.

“The blood of our soldiers is on the hands of those who ordered this murder. Those who created, nurtured and armed these beasts in human form and named them the 'opposition',” he said.

Separately, the defence ministry said a Russian Su-33 fighter jet, based on the aircraft carrier Admiral Kuznetsov, crashed into the Mediterranean Sea after returning from a sortie over Syria.

The ministry said that “because of the failure of the arrester system's cable, the Su-33 fighter rolled off the deck” on Monday.

The pilot successfully ejected and was unharmed, the ministry said, adding that Russian military operations over Syria would not be affected by the incident.

This is the second loss of an aircraft from Russia's only aircraft carrier since it arrived off Syria last month. A MiG-29 crashed into the sea on November 15 while attempting to land on the Admiral Kuznetsov.

In Aleppo, rebel shelling on the government-held part of the contested city has intensified in recent weeks as Syrian government and allied troops push their way into parts of Aleppo controlled by the opposition.

In an offensive that began last week, Syrian government forces seized large areas of the Aleppo enclave that have been under rebel control since 2012.

The fighting was most intense on Monday near the dividing line between east and west Aleppo as government and allied troops pushed their way from the eastern flank, reaching less than half a mile from the citadel in the centre of the city.

Rebel fighters clashed with advancing troops and also fired mortars and shells into the government-controlled part of Aleppo to the west. Syrian State TV said four civilians were killed on Monday in three different neighbourhoods in western Aleppo.

In pictures: Aleppo bombing Show all 14 1 /14 In pictures: Aleppo bombing In pictures: Aleppo bombing Bombing in Aleppo Smoke rises after airstrikes on the rebel-held al-Sakhour neighborhood of Aleppo, Syria April 29, 2016. Reuters In pictures: Aleppo bombing Bombing in Aleppo A Syrian family runs for cover amid the rubble of destroyed buildings following a reported air strike on the rebel-held neighbourhood of Al-Qatarji in the northern Syrian city of Aleppo, on April 29, 2016. AFP/Getty Images In pictures: Aleppo bombing Bombing in Aleppo A man reacts as he stands on blood stains at a site hit by airstrikes in the rebel held area of Aleppo's al-Fardous district, Syria, April 29, 2016. Reuters In pictures: Aleppo bombing Bombing in Aleppo The damage of the airstrikes in the rebel-held area of Aleppo on April 28 Reuters In pictures: Aleppo bombing Bombing in Aleppo The damaged the Medecins Sans Frontieres (MSF)-backed al-Quds hospital after it was hit by airstrikes, in a rebel-held area of Syria's Aleppo Reuters In pictures: Aleppo bombing Bombing in Aleppo Syrians evacuate an injured man amid the rubble of destroyed buildings following an air strike on a rebel-held of Aleppo on April 29, 2016. AFP/Getty Images In pictures: Aleppo bombing Bombing in Aleppo People inspect the damage at a site hit by airstrikes, in the rebel-held area of Aleppo's Bustan al-Qasr AP In pictures: Aleppo bombing Bombing in Aleppo A man leads a woman in tears and child out of the scene after airstrikes hit Aleppo AP In pictures: Aleppo bombing Bombing in Aleppo Civil defence members search for survivors after an airstrike at a field hospital in the rebel held area of al-Sukari district of Aleppo Reuters In pictures: Aleppo bombing Bombing in Aleppo A Syrian boy is comforted as he cries next to the body of a relative who died in a reported air strike in the rebel-held neighbourhood of al-Soukour in the northern city of Aleppo Getty Images In pictures: Aleppo bombing Bombing in Aleppo A Syrian family walks amid the rubble of destroyed buildings following a reported air strike in the Bustan al-Qasr rebel-held district of the northern Syrian city of Aleppo Getty Images In pictures: Aleppo bombing Bombing in Aleppo Syrian civil defence volunteers and rescuers remove a baby from under the rubble of a destroyed building following a reported air strike on the rebel-held neighbourhood of al-Kalasa in the northern Syrian city of Aleppo Getty Images In pictures: Aleppo bombing Bombing in Aleppo Syrians help a wounded youth following an air strike on the Fardous rebel held neighbourhood of the northern Syrian city of Aleppo Getty Images In pictures: Aleppo bombing Bombing in Aleppo Syrian civil defence volunteers evacuate people from a damaged building following a reported airstrike in the rebel-held neighbourhood of Tareeq al-Bab in the northern city of Aleppo

The opposition-run Thiqa News agency and the Syrian Civil Defence in Aleppo city said four civilians were killed in rebel-held Zabadiyeh district when barrel bombs were dropped.

In the nearby rebel-controlled Idlib province, Syrian opposition activists said Russian and Syrian aircraft stepped up assaults, a day after air raids killed more than 60 people.

The activist-run Local Coordination Committees said air strikes hit the towns of Binnish, Maarat Nasaan and Saraqib, as well as the provincial capital Idlib. The network said three children were killed, blaming the attacks on Russian aircraft.

The Britain-based Syrian Observatory for Human Rights said 288 civilians have been killed in the province since October 20, when Syrian government and Russian aircraft intensified air strikes.