The 24-page report by the UN Commission of Inquiry detailed the ordeal many Syrians have faced in the first six months of 2018.
"As pro-government forces moved to recapture large swaths of territory from armed groups and terrorist organisations, over one million Syrian men, women, and children were displaced with most now living in dire conditions," said the report.
It noted combatants on the ground failed to take any action to protect civilians.
"No one is acting according to their responsibilities, human rights wise or otherwise," commissioner Karen Abuzayed told Al Jazeera.
"Everyone is to blame and is following their own interests. It's a disaster for the people who have no way to defend themselves."
Civilians were forced to survive in tents or abandoned buildings in the northwest and living on extremely limited humanitarian aid, the report said.
Since 2014, the Syrian government and armed opposition groups have reached a series of reconciliation agreements in a number of besieged areas, mainly aiming to allow fighters to leave government-surrounded towns for opposition-held areas in Idlib province, which borders to the north.
While the Syrian government positively regards such reconciliation agreements, armed groups and activists, however, view them as "compulsory displacement" aiming to reshape the demographic structure of the country.
With a Russian-backed Syrian government offensive looming, the three-member UN commission also warned against a major attack on Idlib - the last remaining rebel stronghold - and called on all parties to guarantee the safety of the three million civilians there.
The report warned an attack on Idlib "with little regard for civilian life would generate a catastrophic human rights and humanitarian crisis".
The report also detailed the use of chlorine gas three times by Syrian government forces in January and February in the Damascus suburb of Eastern Ghouta, which at the time was under control of opposition armed groups. The two chemical attacks wounded 21 people.
War crimes in Idlib?
President Bashar al-Assad has vowed to retake Idlib province, backed by his Russian and Iranian allies. Syrian government and Russian warplanes began air raids in Idlib last week.
According to the UN Office for the Coordination of Humanitarian Affairs, the escalation in Idlib over the past 10 days has resulted in at least 30,000 people fleeing their homes in rebel-held territories in northwest Syria.
France's foreign minister said on Wednesday the bombing by Russian, Syrian and Iranian forces could amount to war crimes.
"The hypothesis of war crimes cannot be excluded ... once one begins to indiscriminately bomb civilian populations and hospitals," Jean-Yves Le Drian told legislators.
"The situation is extremely serious. We are on the eve of a considerable humanitarian and security catastrophe," he said.
On September 7, a summit in Tehran failed to produce a clear agreement between Russia, Turkey, and Iran on the fate of Idlib.
A ceasefire suggested by Turkish President Recep Tayyip Erdogan was rejected and a full-fledged government offensive now appears imminent, in what is expected to be Syria's deadliest battle yet.
Idlib is the last barrier standing between the Syrian government and its military victory against a rebellion that began in March 2011.
"Idlib should not become the next massacre, the final massacre in the battles in Syria and common sense now needs to prevail," commissioner Hanny Megally told reporters after the release of the report.
Aid workers in Idlib told Al Jazeera that camps are already overcrowded and undersupplied and are not able to properly address the needs of the newcomers.
Tamam Atour, who works with the Turkish humanitarian organisation IHH in the area of Bab al-Hawa near the border with Turkey, said there is a severe lack of basic supplies.
Of the 200-300 families who made it to camps where he works, only 40 were provided with tents. The rest had to sleep out in the open.
"We are suffering from a severe lack of supplies, lack of blankets, food baskets, detergents, kitchen utensils. The large number [of newcomers] is causing us great confusion and we don't know how to deal with them," he said.
Khalid Shalas, manager of one of the camps in the al-Zawf area, said his camp is lacking the means to provide basic needs. He said there is almost no baby formula and little medicine available.
Shalas and his colleagues have sent out an urgent appeal to a number of Western and Arab aid organisations requesting help but have only received negative answers.
"If there is an attack now on Idlib it would produce a humanitarian catastrophe. People will prefer staying in their houses and dying in air strikes over sleeping in the street with no shelter, at the mercy of wild beasts and snakes," Shalas said.
The UN has warned an attack would result in at least 800,000 civilians fleeing to the borders with Turkey, resulting in the worst humanitarian catastrophe of the 21st century.
Mariya Petkova contributed to this report from Antakya, Turkey