It is "highly probable" the Syrian government or its allies carried out attacks on three medical centres, a school and a refuge for children in north-west Syria last year, an internal UN inquiry found.
The inquiry also found it "probable" that a deadly attack on a Palestinian refugee camp in Syria's Aleppo was carried out by armed opposition groups or Hayat Tahrir Al Sham militants.
Syrian President Bashar Al Assad's forces, backed by Russia, began an offensive early last year on the last major insurgent stronghold in north-west Syria.
Russia and Syria have said their forces do not target civilians or civilian infrastructure.
The Syrian mission to the UN did not immediately respond to a request for comment on the summary of the UN report, which Secretary General Antonio Guterres submitted to the Security Council on Monday.
Syria's civil war, a decade of decay
"The impact of the hostilities on civilian and humanitarian sites in north-west Syria is a clear reminder of the importance for all parties to the conflict to observe and ensure respect for international humanitarian law," Mr Guterres wrote to the council.
"According to numerous reports, the parties have failed to do this."
Under pressure from two thirds of the Security Council, Mr Guterres announced in August that the UN would investigate attacks on UN-supported sites and other humanitarian areas in north-west Syria.
The locations of those sites had been shared with the warring parties in a bid to protect them.
But the UN has questioned whether it made them targets.
Mr Guterres noted that the members of the board of inquiry were unable to visit Syria to investigate because the regime did not respond to repeated requested for visas.
The attacks investigated by the board took place in April, May and July.
Fighting has calmed in the north-western region after Turkey, which backs rebels opposed to Mr Al Assad and sent in more troops this year, agreed on a ceasefire with Russia a month ago.
The fighting has displaced nearly 1 million people in Idlib.
A crackdown by Mr Al Assad on pro-democracy protesters in 2011 led to Syria's civil war.
