BEIRUT — It’s an extraordinary plea to the American president from the handful of Syrian doctors still working in rebel-held Aleppo.

In a letter addressed to President Obama, 15 physicians left in the bombed-out eastern areas of the city called Thursday for help in getting humanitarian aid to 300,000 civilians trapped there and ending punishing air raids by warplanes operated by Syria’s government and Russia.

Fighting in the key northern city — divided since 2012 between opposition areas in the east and government-held ones in the west — has raged in recent days. On Saturday, rebel fighters broke and tentatively ended a brutal siege of their strongholds in the city by pro-government forces that had triggered worsening shortages of food and medicine. In response, government-loyal ground troops backed by air support from Russia have been mounting a counterattack there in what appears to be a crucial escalation in the five-year-old Syrian conflict.

In their attacks, government and Russian aircraft have intensified airstrikes on medical facilities in opposition areas across Syria, including eastern Aleppo, according to rights groups and rebel forces.

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