Russia blocked a nine-month extension of a key Syria aid route at the United Nations Security Council on Tuesday, throwing the vital mechanism that provides life-saving support to millions of people into doubt.
The UN-brokered agreement that allows for the delivery of aid overland from Turkey into rebel-held areas of Syria expired on Monday.
While many council members including the United States and Britain have called for a full-year extension, Russia has insisted on just six months.
It vetoed the nine-month compromise proposed by Switzerland and Brazil.
US Ambassador Linda Thomas-Greenfield called the veto "an act of utter cruelty."
The 15 members of the Security Council had been trying for days to find a compromise to extend the deal, which since 2014 has allowed for food, water and medicine to be trucked to northwestern Syria without the authorization of Damascus.
The crossing provides for more than 80 percent of the needs of people living in rebel-controlled areas -- everything from diapers and blankets to chickpeas. The government in Damascus regularly denounces the aid deliveries as a violation of its sovereignty.
Russia has been chipping away at the aid mechanism for years.
The accord originally allowed for four entry points into rebel-held Syria, though now only the Bab al-Hawa crossing remains passable. The aid mechanism comes up for renewal every six months due to pressure from Damascus ally Moscow.