The Syrian government has not granted the United Nations “a single permit” to bring aid to the besieged city of Aleppo as part of a cease-fire that took effect this week, a senior U.N. official said Thursday.

Stalled convoys, seeking to ferry food for tens of thousands of people, have quickly become a symbol of the wider challenges linked to the cease-fire effort even as fighting has sharply eased over the past few days.

Opening channels for humanitarian access is a key provision of the cease-fire plan brokered by the United States and Russia, which are on opposing sides of the war. But the wrangling over the aid — now stuck on the Syria-Turkey border — has led to questions over achieving the broad goals of the accords, beginning with widespread humanitarian relief.

“We could go today. We’re not. . . . The permits have not been given,” Jan Egeland, chairman of the Syrian humanitarian task force, told reporters in Geneva.

“We hope to go tomorrow, to eastern Aleppo,” he said, referring to a rebel-held area that has been under siege by government forces­ for more than a month. Aleppo, however, is just one of many areas cut off from aid and other assistance.

“Not a single permit is in the hands of our people,” Egeland said.

The 40 trucks — carrying rice, wheat and other items — “can go at a moment's notice,” he said.

The cease-fire began Monday and has largely held in key areas of the country. The reduction in violence has given civilians some respite.

But as the fighting subsides, the focus has turned toward the blocked aid.

If the level of conflict is down, there is “no reason, no excuse, for not being able to deliver,” Staffan de Mistura, the U.N. special envoy for Syria, said Thursday.

“Are we disappointed?” he said. “Of course, we are.”

Both U.N. officials laid blame on the Syrian government, which requires layers of permission at the senior and local levels. Aid convoys are often required to display permits at each armed checkpoint into besieged areas.

Since 2011, troops at roadblocks have removed nearly 50 tons of medical supplies from aid convoys, according to the World Health Organization. These include antibiotics, mental health medication and kits to treat burns or help deliver babies.

The United Nations insisted Thursday that the government would not be allowed to pick through any of the aid items headed to Aleppo, about 40 miles south of the Turkish border.

Once the trucks begin to move, “they will not be harassed,” de Mistura said. “No saying: ‘We will take out that medicine, and we will take that food.’ ”

Nearly half a million people have been killed in the years-long civil war, and millions more have been displaced or marooned in embattled areas.

Also of concern was safety along Castello Road, the only way in and out of eastern Aleppo. Under the agreement, the highway would become a demilitarized zone and humanitarian corridor.

Government troops control Castello Road, but opposition groups are also stationed nearby.

The journey from the Turkish border to Aleppo “is not an easy one,” David Swanson, a spokesman for the U.N. Office for the Coordination of Humanitarian Affairs, said Thursday. “Security is a major concern, and road conditions are poor.”

Despite the accord, it was unclear Thursday whether either the government or opposition groups planned to withdraw.

The Russian military, which backs Syria’s government, said Thursday evening that Syrian troops had pulled back from the highway but that opposition fighters had failed to do the same, the Associated Press reported.

Just a few hours earlier, nearby rebels said that they had not withdrawn but that neither had Syrian troops.

The troops “should have withdrawn from Castello Road [on Thursday morning] and the rebels too,” said Zakaria Malahiji, political officer of the Aleppo-based Fastaqim rebel group.

But “the regime troops have not left their positions,” he said, “and neither have our guys, because they do not trust that the regime won’t advance if they retreat from their positions.”

The quarreling over the highway prompted de Mistura to urge the United States and Russia to influence the respective warring parties. The United States has backed some rebel groups and worked with the opposition, while Russia is allied with Syrian President Bashar al-Assad.

Possible friction between Moscow and Washington also cast doubts on the broader attempts to keep the fighting in check.

In Moscow, a spokesman for Russia’s Defense Ministry accused the United States of not fulfilling its obligations under the cease-fire agreement.

The spokesman, Maj. Gen. Igor Konashenkov, told the Interfax news service that rebel troops continued to fire artillery at government forces­ and had not separated themselves from units loyal to a former affiliate of al-Qaeda that both the United States and Russia have deemed a terrorist organization.

Konashenkov also complained about statements by U.S. officials doubting whether Russia would fulfill the terms of the cease-fire.

“There’s an impression that the goal of Washington’s ‘curtain of words’ is an intention to hide the fact that it is not fulfilling its obligations, above all the separation of units of the ‘moderate opposition’ from the terrorists,” Konashenkov said.

A possible next step in the cease-fire would coordinate U.S. and Russian attacks against militant factions including the Islamic State and the former al-Qaeda militia, now called Jabhat Fatah al-Sham.

Andrew Roth in Moscow, Zakaria Zakaria in Istanbul and Louisa Loveluck in Chicago contributed to this report.