Doctors Without Borders pleaded on Monday for access to treat the wounded in the rebel-held part of Syria's Aleppo as government forces pressed ahead with an offensive that has killed hundreds of people in recent weeks.

The international charity, also known by its French acronym MSF, said in a statement that medical workers in Aleppo are exhausted and that the overstretched facilities face an impending fuel shortage. MSF, which supports eight hospitals in Aleppo's besieged eastern quarters, says just 35 doctors remain in the area, serving a population of 275,000.

Eastern Aleppo's Health Directorate said the wounded were sleeping outside overcrowded hospitals, waiting for care. The UN has warned that the Aleppo bombardment by Syrian and Russian warplanes could leave thousands more dead by the year's end.

This frame grab from a Doctors Without Borders video shows a house on fire in Aleppo, Syria. The international charity says medical workers in Aleppo are exhausted and that the overstretched facilities face an impending fuel shortage. (Doctors Without Borders/AP)

"Russia and Syria must stop the indiscriminate bombing now and abide by the rules of war to avoid the extreme suffering of the unprotected civilian population," said Pablo Marco, MSF's operations manager for the Middle East.

The Britain-based Syrian Observatory for Human Rights, which monitors the conflict through local contacts, reported heavy fighting along the east-west Aleppo front lines. Another activist-run group, the Local Co-ordination Committees, said rebels were fighting to repel government forces from the city's largest water facility, which serves over a million people.

In another besieged area near the capital, Damascus, doctors reported up to two dozen cases of kidney failure that they said resulted from malnutrition. Muhammad Darwish, a local physician, said doctors confirmed renal failure in 12 people in the town of Madaya and were investigating another 12 cases.

Government forces have laid siege to Madaya, home to some 40,000 people, since late last year. Last winter, MSF reported at least 16 deaths there resulting from malnutrition and lack of medical care.

"We are only eating carbohydrates. We aren't receiving any vitamins or protein," said Darwish.

UN deadlocked over crisis

The government has prohibited the UN from delivering seeds or dialysis kits to the town, in what the opposition says is a strategy aimed at forcing the town to surrender.

The UN Security Council is deadlocked over how to respond to the Aleppo crisis, with Russia on Saturday vetoing a French-drafted resolution demanding an immediate halt to Russian and Syrian airstrikes and the rest of the council voting down a Russian counter-resolution.

Moscow has been a key backer of Syrian President Bashar al-Assad throughout the conflict.

Russia and Syria's government have both endorsed a proposal by UN envoy Staffan de Mistura to expel al-Qaeda-linked militants from east Aleppo while skirting around the reciprocal part of the proposal, to abide by a ceasefire for the besieged area.

An official at Syria's Foreign Ministry said the government would ensure the safety of any civilians or militants who desire to leave, saying gunmen could take their personal weapons and relocate to other rebel areas. The official spoke to the AP on condition of anonymity, in line with ministry regulations.

Russian President Vladimir Putin said during a visit to Turkey on Monday that Moscow and Ankara want to facilitate the delivery of aid to Aleppo, but that Syrian rebels would have to withdraw from a main supply route to ensure the safety of deliveries. He blamed the U.S. for failing to get them to do so, saying, "the American side does not want to do that, or is not capable of doing that."

Russian President Vladimir Putin seen here at the World Energy Congress, in Istanbul on Monday said that Moscow and Ankara want to facilitate the delivery of aid to Aleppo. (Alexei Druzhinin/Sputnik/Kremlin Pool Photo/AP)

Moscow and Washington have repeatedly traded blame for the failure of a Syrian ceasefire last month.

U.N. chief Ban Ki-moon, meanwhile, urged the Security Council to refer the situation in Syria to the International Criminal Court for investigation of possible war crimes. He told reporters Monday that he was "deeply disappointed" at the lack of unity on Syria in the council, saying it must stop debating and disagreeing and "work to protect human lives, to bring this matter to a political solution."

Earlier, in an interview with the German broadcaster Deutsche Welle, Ban blamed Assad for the bloodshed, saying more than 300,000 people have been killed since the start of the war because of his "failure of leadership." He also referenced previous conflicts in which the international community failed to halt mass killings.

"We should have prevented Srebrenica. We should have prevented the Rwandan genocide. In Aleppo, we're doing our utmost," he said. "The future of one person, like President Assad, should not block this process."

The Czech Republic, meanwhile, dispatched a plane with medical aid to the Syrian Arab Red Crescent. The Czech Republic has co-ordinated aid deliveries directly with the Syrian government, while other Western nations have worked through third parties. The Czechs pledged earlier this year to provide aid worth some $8 million to Syria until 2019.