BEIRUT (Reuters) - Syrian rebels fought fiercely with pro-government forces trying to advance into opposition-held areas of eastern Aleppo and warplanes kept up their bombardment of the area on Friday in a renewed bid by Damascus to retake the entire city.

The U.N. humanitarian adviser said the besieged population of eastern Aleppo faced a “very bleak moment” with no food or medical supplies, winter approaching, and an increasingly fierce attack by Syrian and allied forces.

Violence also escalated in and around Damascus, where government forces bombarded the city’s rebel-held eastern outskirts and rebels fired rockets into the government-controlled city center, witnesses said.

Syrian government forces and allied militia renewed a heavy bombardment of rebel-held eastern Aleppo on Tuesday after a pause of several weeks. Russia, whose air force is bombing in support of Assad, says it has not taken part in the latest Aleppo attack. Moscow has, however, escalated its role in the war, launching attacks on other rebel-held areas from the sea.

Bombs hit a hospital in east Aleppo on Friday evening, the fourth health facility put out of service there in four days, the Syrian Observatory for Human Rights monitoring group and a health official from a rebel area said.

The government, backed by the Russian air force and Shi’ite militias, has this year steadily closed in on eastern Aleppo, first besieging a population estimated by the United Nations to number 270,000 and then launching a major assault in September.

The battle for what was once Syria’s largest city is now the central focus of a five-and-a-half-year-old civil war, which is potentially entering a new phase after the election of Donald Trump as U.S. president. While Trump’s Syria policy has not been fully spelled out, he has suggested Washington could re-examine its longstanding opposition to Russia’s support for Assad.

REBELS DEEPLY ENTRENCHED

Mohamad Abboush, an east Aleppo resident, told Reuters an air strike killed two of his relatives, a 45-year-old uncle and a 12-year-old cousin, on Friday morning. As they sought out medical care for other relatives wounded in the attack, he said they found one hospital in ruins and another in flames.

The air strike had completely destroyed a four-storey apartment block where his relatives had been living in the Tariq al-Bab neighborhood, he said. The survivors had been taken to houses in another area, but nowhere was safe.

“The whole of Aleppo is being bombed,” he said.

Heavy bombardment also took place in the rebel-held area of Eastern Ghouta near Damascus. A witness said shelling and air strikes ongoing since Thursday evening were the worst seen for at least a year.

The Observatory said air strikes and shelling on the Eastern Ghouta town of Douma had in the past 24 hours killed at least 22 people - 10 of them children - and injured dozens. The death toll was expected to rise as many of the injuries were serious.

Slideshow ( 7 images )

Witnesses and the Observatory said rockets fired from rebel areas near the city hit central Damascus on Thursday and Friday. Syrian state media said one woman died and others were injured in rockets fired on residential areas of Damascus on Friday.

While the government side has unleashed enormous firepower, its advances into rebel-held areas of Aleppo since September have been limited. The rebel forces are deeply entrenched and say they are well prepared for urban war.

Fighting was reported on the southern and eastern periphery of the rebel-held area on Friday. The Observatory said Friday’s clashes were the fiercest in Aleppo this week.

Slideshow ( 7 images )

Sources on both sides said pro-government militias were mobilized in large numbers.

An official in the Levant Front rebel group, which fights under the Free Syrian Army banner, told Reuters that the pro-government forces appeared to be seeking to advance along a highway that bisects the rebel-held part of Aleppo.

His group lost a commander along with a number of his men in the fighting, he said. “The militias are coming in strongly in the areas they are trying to storm. There are few frontlines in Syria in general at the moment, most of the focus of the regime and militias is in Aleppo,” he said.

A source on the government side confirmed large mobilization by pro-Assad forces. The source said the shelling of recent days was in preparation for ground operations. A media unit run by Hezbollah, a Lebanese Shi’ite group fighting in support of Assad, said the army had made progress in northeastern Aleppo.

Rebels repelled an attempt by pro-government militias to advance in the Sheikh Saeed area on the southern periphery of eastern Aleppo after heavy bombardment, Al-Farouk Abu Bakr, a commander in the Ahrar al-Sham group, told Reuters. Syrian military officials could not immediately be reached for comment.

In the four days since bombardment resumed on eastern Aleppo, the Observatory says it has documented 65 deaths and hundreds of injuries in east Aleppo, and four deaths and dozens of injuries in government-held west Aleppo as a result of rebel rocket fire. State media said shelling killed five more people in western Aleppo on Friday.

Hundreds of people have been killed in the east Aleppo assault since September, according to U.N. figures and rescue workers. Rebel shelling, meanwhile, killed dozens of people in government-held western Aleppo during a failed counter attack.

The Observatory said government forces had targeted areas near three hospitals to keep them out of service. The government has previously denied such accusations.

U.N. humanitarian adviser Jan Egeland said rebel groups in Aleppo had agreed in principle to a U.N. humanitarian relief plan that allows medical workers, medical supplies and food into eastern Aleppo and enables the evacuation of the sick and wounded, but operational details had yet to be agreed.

Egeland said Russia had given positive signals about the plan but had not given an official green light. The United Nations had hoped to send convoys with aid for 1 million Syrians in besieged or hard-to-reach areas this month, but so far not one has reached its destination.