Eight children under five years of age have died within days in a northeastern Syria camp hosting thousands of relatives of jihadists, the Save the Children charity said.
Health and nutrition services have deteriorated rapidly in the Kurdish-run Al-Hol camp, where the children died over a five-day period, it said.
"The spike in under-five mortality was recorded between 6-10 August and is more than three times higher than the mortality rate since the beginning of 2020," a statement said.
The Al-Hol camp is home to tens of thousands of people, including the relatives of Islamic State group jihadists.
It is run by the autonomous Kurdish administration that holds most of northeastern Syria and has reported 171 COVID-19 cases, including 8 deaths in areas under its control.
Medical assistance to Al-Hol has decreased since the UN Security Council in January scrapped a key border crossing used to deliver UN-funded medical aid to camp residents.
"We are seeing a collective failure at all levels to protect children," said Save the Children Syria Response Director Sonia Khush.
"This is the result of ongoing failure of the UN Security Council to reopen the closest border-crossing, leading to unforgivable delays in services."
The charity said the deaths of the eight children in August were linked to heart failure, internal bleeding and severe malnutrition -- all of which could be treated in operational field hospitals.
It said border crossing restrictions had reduced capacity of operational health facilities in Al-Hol by 40 percent.
The crisis has been compounded by the coronavirus pandemic in the overcrowded camp and has sparked fears among aid groups of a health catastrophe.
The United Nations on August 6 said that three health workers in Al-Hol had tested positive for coronavirus.
Two days later, Save the Children said it recorded the first case among camp residents, warning the disease would spread amid reduced medical access.
On Thursday the UN's Humanitarian Coordinator for Syria, Imran Riza, also expressed alarm over the child deaths.
"It underscores the basic fact that no child should be forced to live under the challenging and potentially dangerous humanitarian conditions at Al Hol camp," he said in a statement.