The closure of the last aid corridor from Turkey into northwest Syria's opposition-held areas would spell "catastrophe" for millions of people, a United Nations aid official has warned.

"This is one of the most vulnerable populations anywhere in the world," said Mark Cutts, U.N. deputy regional humanitarian coordinator for the Syria crisis. "It is absolutely essential that we keep this lifeline going."

Cutts spoke ahead of a U.N. Security Council vote to renew the world body's authorization to deliver assistance through the Bab al-Hawa crossing before its mandate expires on July 10.

More than 4,600 aid trucks, carrying mostly food, have crossed it so far this year, helping some 2.4 million people, says the U.N. Office for the Coordination of Humanitarian Affairs (OCHA).

Russia, an ally of the Bashar Assad regime, has threatened to veto the proposal to extend the aid mechanism having already forced a reduction in the number of crossings, arguing that it violates Syria's sovereignty.

"We know things this year are even more politicized than in previous years," Cutts told Agence France-Presse (AFP). "The tensions are very high with the war in Ukraine."

But he warned that a "failure to renew this resolution will be a catastrophe. There is no alternative currently available that can replace the scale or scope of what the U.N. is currently doing."

Syria's humanitarian needs have reached their highest levels since the 2011 onset of a bloody conflict, that has killed nearly half a million people and forced more than half of the country's prewar population from their homes.

About 13.4 million people across Syria were in need of assistance last year, up from 11.1 million in 2020, OCHA says.

The U.N. resolution permitting aid deliveries across the Syrian-Turkish border at Bab al-Hawa has been in effect since 2014.

Syria's opposition-held northwest is home to more than 4 million people, most of whom are displaced and live in poverty.

Aid groups are considering alternatives in the event of a Russian veto, relief officials told AFP on condition of anonymity.

They include stepping up deliveries via Damascus and continuing cross-border deliveries through a consortium of international aid groups.

Cutts declined to speak about the U.N.'s contingency plans, but said that they could not substitute the cross-border operation that is heavily involved in risk mitigation, monitoring and reporting.

"Our focus is always on ensuring that the aid reaches the people who need it, and that it is not diverted to armed group," Cutts said.

"Without the U.N. role, there will be less accountability and less transparency in the overall response ... It's hard to guarantee what the situation will be."

Russia argues that aid can reach needy populations through regime-held areas within Syria, but Cutts said the U.N. has only managed to arrange for five such deliveries so far.

"We are trying to have as much access as possible from different routes," Cutts said. "But it remains a war zone and the cross-line access is always dependent on cooperation between parties to the conflict."

Amnesty International on Tuesday accused the Syrian regime of deliberately leaving displaced Syrians in areas outside of its control entirely dependent on international aid.

"Since losing control of the northwest part of the country, the Syrian government has cut off electricity and water supplies, obstructed aid and attacked camps, medical facilities and schools, putting the onus on humanitarian organizations to provide services," the rights group said in a statement.

"There is no effective solution for providing adequate humanitarian aid in northwest Syria except by renewing the existing cross-border mechanism."