UNITED NATIONS >> Access to food and medicine, a recurrent weapon of war in Syria, has emerged again as a political flash point: The Syrian government’s most powerful ally, Russia, is balking at the proposed renewal of a United Nations measure that allows aid deliveries to pass through Syria’s borders into rebel-held areas of the country, without permission from government authorities.

Russia’s ambassador to the United Nations, Vassily Nebenzia, has said such deliveries, authorized by a Security Council resolution that expires in January, impinge on Syria’s sovereignty.

The U.N. emergency relief chief, Mark Lowcock, has called cross-border aid shipments “essential to save lives.” Over the years, U.N. aid officials have chronicled how hard it has been to get permission from Syrian government authorities to cross front lines and deliver food, blankets and medicines to people living in areas under opposition control.

The debate over cross-border aid comes as the government of Syria’s president, Bashar Assad, with military backing from Russia, has reclaimed a great deal of the territory it lost over nearly seven years of war. With the battlefield reshaped, international aid groups that had worked in opposition-held areas are now weighing how to serve those very same communities that are now under government control.

It is a delicate issue for aid organizations. The Syrian government has long insisted that organizations wanting to work in government-held areas cannot work in rebel-held territory. It threw out one large aid group, Mercy Corps, in 2014, for doing so. Aid workers say that such restrictions violate humanitarian principles, and that they must be allowed to work in areas where needs are greatest.

Representatives of six aid groups spoke about the choices they have been weighing in recent weeks — all on condition of anonymity in an effort not to doom their chances of obtaining government authorization.

All the representatives insisted that it was essential to continue cross-border operations because the only other way to reach those in need is to cross front lines, from government to rebel-held territory, and Damascus has consistently put up obstacles to cross-line convoys. In report after report, the United Nations has documented bureaucratic delays and the removal of goods from humanitarian convoys, including medicines and medical supplies.

The bulk of U.N. aid to Syria is delivered to government-controlled areas, with various agencies providing food, medical care and school supplies — even garbage removal in some newly reclaimed areas — for 2.8 million Syrians. An additional 800,000 Syrians received U.N. aid through cross-border convoys in November. “This sustained assistance is essential for those in need,” Lowcock said.

Aid trucks are inspected and sealed at the border and their contents distributed by local aid groups as part of an arrangement authorized by the 2014 Security Council resolution, designed to ensure that aid could be distributed where it was needed. The resolution was renewed twice, with Russian support. This time, though, Russia says it wants tighter inspections, according to diplomats, but has yet to propose anything specific.

Last month it vetoed the renewal of a U.N. panel to investigate the use of chemical weapons in Syria.

“This mechanism cannot remain as it presently stands,” said the Russian ambassador, Nebenzia, in his only public comments about the cross-border aid resolution.

Negotiations are expected in the coming days, with Japan and Sweden, both temporary members of the Council, leading the talks.

“We cannot accept any changes that would hamper this lifesaving assistance to continue,” Carl Skau, Sweden’s deputy ambassador, said, adding that his country was open to discussing “ways to further enhance transparency.”