(CNN) A nervous calm appears to have descended on Syria on the first full day of a ceasefire, but aid has not yet been able to reach besieged populations.

Aid agencies stand poised to distribute much-needed assistance but say they are awaiting guarantees of security from all parties before beginning their deliveries to hundreds of thousands of desperate Syrians.

The Syrian Foreign Ministry says it refuses any entry of humanitarian aid to the city of Aleppo, a priority for aid agencies, unless it is coordinated through the Syrian government and United Nations -- especially aid coming from Turkey, the state-run Syrian Arab News Agency reported.

Staffan de Mistura, UN special envoy for Syria, told reporters in Geneva, Switzerland, that there has been a "significant drop in violence" and that the "situation has dramatically improved, with no airstrikes."

He said humanitarian aid has not yet been delivered because the Syrian government hasn't sent a letter of authorization to the United Nations.

International Committee of the Red Cross spokeswoman Krista Armstrong said supplies were in warehouses ready to be delivered to rebel-held east Aleppo and other besieged areas as soon as they were cleared to enter. Between 250,000 and 275,000 people in east Aleppo have been cut off from assistance since early July, according to the United Nations.

But she said that "there hasn't been a breakthrough in accessing new areas" without any guarantees on security.

Jens Laerke, deputy spokesman for the UN humanitarian office, said the United Nations called on all parties to respect the cessation of hostilities "to enable unimpeded, unconditional and sustained access without delay."

"We hope and are prepared to commence those deliveries as soon as we are satisfied that the cessation of hostilities holds and that safe deliveries can be made," he said.

The first planned deliveries will provide food assistance to east Aleppo, delivered across the border from Turkey, he said.

Ceasefire coincides with Muslim holiday

The peace appeared to be holding Tuesday, with no major violence reported on the first full day of a hard-won ceasefire brokered by the United States and Russia.

Both the Syrian government and opposition groups reported sporadic incidents in the first few hours after the ceasefire went into effect at sundown (11.45 a.m. ET) Monday.

One monitoring group reported five violations occurred in and around Aleppo in the ceasefire's first hours, while state-run news agency SANA said rebels were responsible for a number of breaches in Aleppo and Homs.

But such violations are not uncommon in the early hours of a ceasefire, and eventually, the weapons largely fell silent.

Residents of Aleppo, Idlib and Homs, accustomed to the buzz of regime military jets overhead, told CNN that the warplanes had vanished from the skies in the hours following the ceasefire, with only the occasional mortar to be heard.

The ceasefire's start coincides with the beginning of Eid al-Adha -- a holiday that commemorates when Ibrahim (Abraham in the Old Testament) prepares to sacrifice his son as God commanded, but God intervenes and stops him at the last moment.

The Feast of the Sacrifice celebrates the value of human life. It follows a weekend during which more than 90 people -- including 28 children -- were killed in airstrikes.

The terms of the deal

The deal calls for a halt to the violence between the Syrian regime and rebel forces, but does not cover militant groups considered terrorists, such as ISIS and Jabhat Fateh al-Sham , formerly known as al Nusra Front.

It is hoped the ceasefire will provide a window for the delivery of aid to the hundreds of thousands of Syrians in desperate need of food, medicine and other essentials.

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Under the terms of the deal, if the peace holds for seven days, then Russia and the United States will begin coordination to target terror groups in the conflict.

US Secretary of State John Kerry on Monday said while the start of the ceasefire looked good, "it is far too early to draw any definitive conclusions."

Just hours before the ceasefire started, Syrian President Bashar al-Assad gave a defiant message to the country's opposition forces, vowing "to retake every piece of land from the terrorists."

Assad's family has ruled Syria for 45 years. He has often referred to opposition members seeking his ouster as terrorists.

Kerry: 'This is less than perfect'

The ceasefire is the second such concerted attempt to bring peace to Syria this year. The United States and Russia coordinated a partial ceasefire in February, but violence soon resumed.

In the wake of the earlier failures, Kerry addressed criticism that the latest deal is flawed.

"Sure, this is less than perfect," he said. "But flawed compared to what? Compared to nothing?

"This catastrophe developed step by step, folks, and it can only be reversed step by step. This is the best thing we could think of."

The Syrian civil war has killed hundreds of thousands of people and forced more than 5 million to flee the country, spawning an international refugee crisis.

A Syrian man carries a wounded child in eastern Aleppo last month.

The Syrian Observatory for Human Rights, a UK-based monitoring group, estimates that up to 430,000 people have been killed in the conflict, although an accurate estimate is almost impossible to obtain.

The group says it has documented 301,781 deaths in Syria, from March 18, 2011, until September 12, 2016, when the latest truce went into effect. But the group's founder, Rami Abdulrahman, said the actual number of deaths is closer to 430,000.

Among the documented deaths were 86,692 civilians -- 15,099 children under 18 and 10,018 women, he said.

The dead included more than 52,000 fighters from other countries, he said.

A statement from US Central Command acknowledged that some civilians appear to have been killed recently in strikes against ISIS targets in Syria. In two of the three incidents a "nonmilitary" vehicle entered the strike zone after an aircraft fired on the targets. In another strike near Raqqa, some civilians who were near the target may have been killed, CENTCOM said.