Syria’s government forces on Saturday pounded rebel-held Aleppo with air strikes and artillery fire killing 27 people, in the worst airstrike since the bombarding resumed earlier this week.
The US has condemned the "heinous actions" of Damascus and its ally Russia, calling for an end to the bombings.
Saturday was the fifth day of renewed assaults by Syrian warplanes on eastern Aleppo districts, a rebel-held enclave of 275,000 people. The onslaught began Tuesday, when Syria's ally Russia announced its own offensive on the northern rebel-controlled Idlib province and Homs province in central Syria.
The bombing on Saturday came after a day of airstrikes that hit four hospitals in east Aleppo. A statement issued late Friday by the opposition's Aleppo Health Directorate said that all hospitals in east Aleppo are out of service because of the bombing over the past days.
"The intentional destruction of infrastructure for survival has made the besieged steadfast people, including children, elderly and men and women, without medical facilities to treat them," the statement said.
The Britain-based Syrian Observatory for Human Rights said not all hospitals in east Aleppo neighborhoods are out of service but people are finding difficulties reaching them because of the intensity of the shelling.
The White House meanwhile demanded an immediate halt to Syrian strikes on eastern Aleppo. White House National Security Adviser Susan Rice said the U.S. is tracking reports about health conditions.
Speaking in Peru, Rice said the U.S. condemns the "horrific attacks" against hospitals and aid workers "in the strongest possible terms". Rice added there was "no excuse" for the attacks.
The White House is putting the onus on Russia to lower the violence and help humanitarian aid get to besieged Syrians. It says President Barack Obama joins other leaders in Europe, and those gathering for an Asia economic summit in Peru over the weekend, in demanding a halt to bombings.
Opposition activists said Saturday's death toll has been the worst since the aerial campaign resumed on Tuesday. Residents said hundreds of artillery shells and dozens of airstrikes have hit the city, increasing the misery of its residents who have been suffering from lack of food and medicine because of the siege imposed by government forces and their allies in July.
"Aleppo is being wiped out in front of the eyes of the world," medical official Mohammed Abu Rajab said in an audio message to The Associated Press from inside the city. "It's not only hospitals that are out of service. All liberated areas in Aleppo are out of service."
"Entire buildings have been completely destroyed," Abu Rajab said.
The Observatory said Syrian government warplanes and artillery struck more than 20 neighborhoods in east Aleppo killing 27 people and wounding many others.
The Aleppo Media Council, an activist collective, said 20 people, including children, were killed in Saturday's violence in the country's largest city and former commercial center.
Pro-government media, meanwhile, reported rebel shelling on government-held parts of the city, saying they killed two and wounded others.
The latest deaths raise to more than 130 the number of people killed in northern Syria since Tuesday.
The United Nations said in a statement released in Damascus that it "is extremely saddened and appalled by the recent escalation in fighting" in several parts of Syria, calling on all sides to cease indiscriminate attacks on civilians and civilian infrastructures.
It urged access to east Aleppo, where more than 250,000 people have been under siege for nearly four months.
It said the U.N stands ready to assist people in east Aleppo "as soon as access is granted by all parties." It said it has shared with all parties to the conflict in Aleppo and concerned member states a detailed humanitarian plan to provide urgently needed assistance to the inhabitants of east Aleppo, and conduct medical evacuations for the ill and injured" confirmed both Syria Humanitarian Coordinator Ali Al-Za'tari and the Regional Humanitarian Coordinator for the Syria Crisis Kevin Kennedy.
"It is imperative all parties agree to the plan and allow us to secure immediate, safe and unimpeded access to provide relief to those most in need in east Aleppo, but equally in all other parts of Syria," they said.
Meanwhile, an aircraft believed to be American killed late Friday a senior official with al-Qaida's affiliate in Syria known as Fatah al-Sham Front northwest of the country along with two other people, according to the Observatory and jihadi websites.
A militant website and jihadis said the man was Egyptian citizen Abu Abdullah al-Muhajer, an Afghanistan fighting veteran who has been recently working closely with Fatah al-Sham Front's leader Abu Mohammed al-Golani.
Al-Muhajer, whose real name was Abdul-Rahman Ali, joined the early jihad and then subsequent infighting in Afghanistan, according to the website, which provided a profile. It said he went on to Pakistan to study for a post-graduate degree in Sharia law and fled again to Afghanistan after being sought by Pakistani police. He worked as a preacher there and was close to the late al-Qaida leader Osama bin Laden and his successor Ayman al-Zawahri. After the U.S. invasion of Afghanistan, he fled to Iran and spent several years in prison before he was released.
The Observatory confirmed that an Egyptian commander with the al-Qaida affiliate was killed along with two people when their vehicle was struck with a missile fired from an aircraft.
The U.S. has killed several top al-Qaida officials in Syria since the start of 2015. Last month, a drone attack killed top Egyptian al-Qaida veteran Ahmed Salama Mabrouk.
(With AP inputs)