Two Russian nurses in a makeshift hospital in Aleppo have been killed by rebel shelling of the Syrian government-held part of the city, officials said.
The deaths 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 nurses also wounded 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 deaths of the Russian medics in Aleppo.
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.
Maj Gen Konashenkov also called on international organisations such as the Red Cross to condemn the attack on the medics "who were doing their medical duty to help the civilians of Aleppo".
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.
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.