The last doctors in the rebel-held east of the Syrian city of Aleppo have urged US President Barack Obama to come to the aid of the 250,000 civilians there.
A letter signed by 15 physicians warns that if attacks on medical facilities continue at their present rate, there could be none left within a month.
US inaction against crimes by Syria's government and its ally Russia means it bears equal responsibility, it argues.
Russian forces, meanwhile, were due to halt actions in Aleppo to allow in aid.
All military activity, air and artillery strikes would be suspended between 10:00 (07:00 GMT) and 13:00 (10:00 GMT) each day from Thursday, a defence ministry official said.
However, the United Nations said three hours would be insufficient to help the millions in need and appealed for 48-hour pauses.
Fighting has escalated in recent days, with rebels severing the government's main route to the west of the city.
The offensive sought to break a siege by pro-government forces, who encircled the east in July with the support of Russian aircraft.
The letter, signed by 15 of the doctors remaining in eastern Aleppo, says that five years into the uprising against President Bashar al-Assad, doctors have "borne witness as countless patients, friends and colleagues suffered violent, tormented deaths".
"The world has stood by and remarked how 'complicated' Syria is, while doing little to protect us. Recent offers of evacuation from the regime and Russia have sounded like thinly veiled threats to residents - flee now or face what fate?"
They say that in the past month there have been 42 attacks on medical facilities in Syria, 15 of them on hospitals where they work.
"Two weeks ago, four newborn babies gasping for air suffocated to death after a blast cut the oxygen supply to their incubators. Gasping for air, their lives ended before they had really begun," they recall.
The doctors warn President Obama: "Continued US inaction to protect the civilians of Syria means that our plight is being wilfully tolerated by those in the international corridors of power. The burden of responsibility for the crimes of the Syrian government and its Russian ally must therefore be shared by those, including the United States, who allow them to continue."
Unless a permanent lifeline to rebel-held Aleppo is opened, they add, "it will be only a matter of time until we are again surrounded by regime troops, hunger takes hold and hospitals' supplies run completely dry."
"We do not need tears or sympathy or even prayers, we need your action. Prove that you are the friend of Syrians."
An earlier statement sent to the media had said there were 29 signatories to the letter, and also quoted it as calling on Mr Obama to impose a no-fly zone over Aleppo.
On Monday, the UN said countless civilians had been killed or injured over the past few weeks in the city, and that the targeting of hospitals and clinics had continued unabated.
Moreover, attacks on civilian infrastructure had left more than two million people without electricity or access to the public water network for several days, it added.