BEIRUT -- The United Nations faces "a problem" in shipping humanitarian aid into Syria, the UN envoy for the war-torn country said Thursday, pinning the blame on the lack of authorization from Bashar Assad's government that has even disappointed Russia, the Syrian president's key backer.

Staffan de Mistura said a U.S.-Russia-brokered ceasefire deal agreed on last week has largely reduced the violence since it came into effect on Monday, but the humanitarian aid flow that was expected to follow has not materialized.

De Mistura said 40 aid trucks are ready to move and that the UN would prioritize delivery to the embattled, rebel-held eastern neighbourhoods of the northern city of Aleppo.

However, the Syrian government has not provided needed "facilitation letters," or permits, to allow for the start of the convoys, de Mistura said. He said the government had agreed on Sept. 6 -- before the ceasefire deal was inked -- to allow aid into five areas, but the authorizations still haven't come.

Aside from the reducing the bloodshed, the "second dividend" of the U.S.-Russia deal is humanitarian access, de Mistura told reporters in Geneva. "That is what makes a difference for the people, apart from seeing no more bombs or mortar shelling taking place."

"On that one, we have a problem," he added. "It is particularly regrettable ... These are days which we should have used for convoys to move with the permits to go because there is no fighting."

"The Russian Federation is agreeing with us," he said.

Jan Egeland, the top humanitarian aid official in de Mistura's office, said the "good news" from the cessation of hostilities was that the bloodshed has dropped -- and that "attacks on schools, attacks on hospitals have stopped."

The "bad news," he said, was a lack of a green light for UN trucks to cross front lines.

"Our appeal is the following -- it's a simple one," Egeland said. "Can well-fed, grown men please stop putting political, bureaucratic and procedural roadblocks for brave humanitarian workers who are willing and able to go to serve women, children, wounded civilians in besieged and crossfire areas."

"If they do that, we're willing and able to go to all these places in the next few days -- and we are very hopeful that we will indeed be able to do so," he added.

Aleppo had been the centre of fighting over the past months and Syrian government forces and their allies launched a wide offensive earlier this month, capturing several areas south of the city and putting eastern rebel-held neighbourhoods under siege.

Over 2,000 people were killed in 40 days of fighting in Aleppo until the ceasefire went into effect Monday. The dead include 700 civilians, among them 160 children, according to the Syrian Observatory for Human Rights. .

"No aid has arrived in Aleppo. The regime is refusing to allow aid into Aleppo," said Aleppo-based activist Baraa al-Halaby.

Earlier on Thursday, activists said the ceasefire was still holding despite some violations. The Observatory said government forces and opposition fighters were ready to withdraw from the Castello road, a main artery into Aleppo, to hand it over to Russian troops.

It said government forces will not start pulling out until the rebels begin to do the same.

Syria's state news agency SANA said opposition fighters opened fire at a location along the Castello road that was being prepared for Syrian Arab Red Crescent representatives. The report said two people guarding the location were wounded.

SANA also reported violations of the ceasefire in the northwestern villages of Foua, saying sniper fire by insurgents wounded a Syrian boy there. It also said three shells were fired at the government-held southern village of Hadar.

The opposition reported 29 violations by government forces Wednesday, including shelling, air raids and heavy machine-gunfire.

Meanwhile, in the eastern province of Deir el-Zour, clashes and shelling over the past 24 hours between government forces and the Islamic State group in the provincial capital, also called Deir el-Zour, killed at least three people, including a child, according to activists and state media.

Elsehwere in the same province, an airstrike Thursday on the IS-held town of Mayadeen killed at least four people and wounded dozens, said opposition activists and Deir el-Zour 24, an activist collective. The Observatory said the airstrike killed seven people. It wasn't known who carried out the airstrike.

The truce does not include the Islamic State group. The U.S.-led coalition, Russia and the Syrian government have been carrying out air raids against the extremist group.

Also not included in the truce is the al Qaeda affiliate in Syria, the Fatah al-Sham Front which was formerly known as the Nusra Front.

Keaten reported from Geneva