Humanitarian chief Stephen O'Brien, addressing a meeting of the foreign affairs committee on Tuesday, said that an estimated 975,000 people, including 100,000 children, were trapped in Aleppo and other besieged towns in Syria.
Describing Aleppo as the "apex of horror", the former British MP painted a grim picture of the current plight afflicting "tens of thousands" of Syrian refugees.
He told the committee, "Life expectancy in Syria has dropped by 20 years, half the population have been displaced and 80 per cent of the people live in poverty."
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He added, "Tonight, some 975,000 Syrians will be living under siege in the country. These are civilians who are being bombed and find themselves isolated. They have to either submit or flee."
O'Brien, the UN under-secretary general for humanitarian affairs and emergency relief coordinator, said, "This is a monstrous situation and the callous carnage we are witnessing is obscene. The world, including here in Europe, surely has a moral duty to act."
Aleppo was Syria's largest city and its commercial and industrial hub before the uprising against President Bashar al-Assad began in 2011.
It has been divided in roughly two for the past four years, with the government controlling the west and rebels the east.
But in the past year, Syrian troops have broken the deadlock with the help of Iranian-backed Shia Muslim militias and Russian air strikes.
O'Brien, who was taking part in a joint discussion between two parliamentary committees, foreign affairs and development, told MEPs that "despite overwhelming odds," UN aid workers had managed to get assistance through to those who were trapped in "medieval-type" conditions.
He said, "On Monday, for example, our heroic teams managed to get aid through to 60,000 people in four besieged towns."
He also highlighted the ongoing refugee crisis in Europe, pointing out that some 300,000 Syrians had fled the conflict during 2016.
"These refugees may place a burden on Europe but this pales when compared with the number of Syrian refugees who have fled to Turkey, Lebanon and Iraq."
Calling for a "cessation of all violence and an end to these medieval sieges", the official said, "The international community can, and must, do better."
The meeting discussed the political and humanitarian situation in Syria.
O'Brien's comments were shared by Staffan De Mistura, UN special envoy on Syria, who told the joint committee hearing that 12 countries and 98 different factions were fighting in Syria.
The situation had "unravelled," he said, since the breakdown of the ceasefire and an increase in military activity.
Turning to the upcoming Donald Trump presidency in the United States, he said, "We have to wait and see what his foreign policies will be but, according to his statements, he says he wants to make the fight against Islamic State a top priority and that is of course welcome and is not something we would disagree with, of course.
"But what about defeating, not just fighting, ISIS? To do that we are going to need an inclusive new form of government in Syria.
"ISIS feeds on the anger, frustration and disappointment of communities in Syria so what is needed is a political formula and that is regardless of whether there's a military solution."