United Nations Secretary-General Antonio Guterres has appealed to the Security Council to extend its approval of aid deliveries from Turkey to millions of people in need in northwest Syria, telling the body: "We cannot give up on the people of Syria".
The UN mandate, which has allowed deliveries from Turkey to Syria's opposition-controlled northwest, expires on July 10.
Syria's ally Russia argues the long-running operation violates the Middle Eastern nation's sovereignty and territorial integrity.
It says more aid should be delivered from inside the country, raising opposition fears food and other aid would fall under government control.
Guterres said in the past year the United Nations had carried out five such deliveries - known as cross-line - into the opposition-controlled northwest, but it was not "at the scale needed to replace the massive cross-border response".
"I strongly appeal to the members of the council to maintain consensus on allowing cross-border operations," he said.
"It is a moral imperative to address the suffering and vulnerability of 4.1 million people in the area who need aid and protection."
Guterres said 80 per cent of those in need in northwest Syria are women and children.
Some 800 trucks a month deliver aid from Turkey under the UN operation, which Guterres asked be extended for another year.
The US Ambassador to the United Nations, Linda Thomas-Greenfield, who visited the Turkish border crossing earlier this month, told the council it had to make a "life or death decision" and that more aid, not less, was needed.
"Cross-line aid alone cannot come close to meeting the dire needs on the ground. It can reach thousands, but not millions," she said.
"Much more help is needed."
In 2014, the Security Council authorised humanitarian aid deliveries into opposition-held areas of Syria from Iraq, Jordan and two points in Turkey.
But Russia and China have whittled that down to just one Turkish border point.
Russia's Deputy UN Ambassador Dmitry Polyanskiy described as "pitiful" the efforts to deliver aid to the northwest of Syria from within the country.
China's UN Ambassador Zhang Jun said the cross-border aid operation was an "extraordinary arrangement" and a timeline needed to be agreed to end it and transition to deliveries from within the country.
"Can anyone, who respects human life and who respects the fundamentals of the UN Charter, afford to disrupt such a vital system?" Turkey's UN Ambassador Feridun Hadi Sinirlioglu asked.