Air attacks have escalated significantly since Turkey shot down a Russian warplane along the Turkey-Syria border on Nov. 24, the aid agencies say, with Russia responding to the incident by stepping up its effort to crush the anti-government rebellion in the insurgent-held provinces bordering Turkey.

Among the targets that have been hit are the border crossings and highways used to deliver humanitarian supplies from Turkey, forcing many aid agencies to halt or curtail their aid operations and deepening the misery for millions of people living in the affected areas, according to a report this month by the U.N. Office for the Coordination of Humanitarian Affairs.

Hospitals and health facilities also have been struck, reducing the availability of medical care for those injured in the bombings. According to the U.N. report, at least 20 medical facilities have been hit nationwide in Syria since Russia launched its air war on Sept. 30.

“This is an emerging humanitarian crisis. There is extreme suffering, and people are not being protected,” said Rae McGrath, country director for Turkey and North Syria for the American aid agency Mercy Corps, one of the largest providers of food aid in northern Syria. Since the Russian strikes began, the agency has been able to deliver only a fifth of the amount it normally provides, he said.