Two major humanitarian successes provided the lone bright spots in the darkening landscape. In one of the largest evacuations from besieged areas, 515 sick and wounded people were extricated from four blockaded areas — two opposition-held towns, Madaya and Zabadani and two government-held towns, Fouaa and Kfarya. Those towns have had more luck in negotiations because they have been treated as a group in talks often involving the government’s ally, Iran, and the insurgents’ Turkish backers.

And a United Nations convoy was reported to be on the way to Rastan, in Homs Province, which has not received aid since 2012.

Even so, the United Nations special envoy for the Syrian conflict, Staffan de Mistura, said on Thursday that aid was reaching only about 560,000 people, out of the 4.1 million believed to be in need in besieged and hard-to-reach areas. United Nations officials say the Syrian government is not allowing medicines or medical equipment to be included in the aid deliveries. “This is not only worrisome, but unacceptable according to international law,” Mr. de Mistura said.

American officials expressed concern over reports that Russia was moving artillery into place around the divided city of Aleppo to help its ally, the Syrian government, begin a new attempt to retake rebel-held areas there. There are growing fears about an offensive unleashing a new wave of refugees toward the Turkish border. They would join the tens of thousands already marooned on the Syrian side.

Those refugees set off a debate Thursday in Brussels over Turkey’s request for visa-free travel for its people across most of the region by early summer. Allowing up to 80 million Turks to visit most countries of the European Union is one of the most divisive parts of a deal the bloc signed last month with Ankara. For now, the agreement has stanched the bloc’s worst migration crisis since World War II by keeping the refugees in Turkey.