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By Cassandra Vinograd

Aid convoys were on standby Tuesday, waiting to see if Syria's new cease-fire agreement would hold.

More than 20 hours after going into effect, officials said the U.S.-Russia brokered deal appeared largely intact. Activists and monitoring groups reported some minor skirmishes but no major violations.

No civilian deaths have been recorded in the first 15 hours of the cease-fire, according to the Britain-based Syrian Observatory for Human Rights.

"The majority of Syrian provinces have witnessed silence during the last night," it said in a statement.

That echoed a cautious initial assessment from Secretary of State John Kerry, who helped broker the deal and said early reports pointed to a "reduction" in violence.

The cease-fire was the result of lengthy negotiations between the U.S. and Russia.

Across Syria both regime forces and rebels are expected to hold fire for a week to allow in much-needed humanitarian aid. Syrian forces, though, can continue launching airstrikes against ISIS and al Qaeda-linked fighters.

However, details on the terms of the deal have not been publicized — though Russia has said it will seek to release them — and not all rebel groups agreed to the cease-fire.

That has raised concerns this agreement could go the way of other cease-fires by unraveling, and before aid can reach war-weary Syrians in besieged areas. The divided city of Aleppo is of particular concern.

Special report: Aleppo's Children

Kerry has said it could take a "day or two" before humanitarian assistance is deployed, noting that U.N. agencies were "preparing" for deliveries.

"There will undoubtedly be reports of a violation here and there … But that is the nature of the beginning of cease-fires almost always," he told reporters Monday shortly after the deal went into effect. "It is important — a very important part of this equation — that access to humanitarian goods take place."

No deliveries have been made as of Tuesday at noon local time, according to the U.N.'s humanitarian aid coordinator.

"We need to have actual concrete assurances — we need to be satisfied — that we are not sending people, aid convoys, into mortal danger. And we don't have that yet," OCHA spokesman Jens Laerke told NBC News.

He said there are "a lot of variables" to take into consideration before the convoys are deployed with aid that "everybody in Syria — particularly around Aleppo — so desperately need."

That means ensuring not just the safety of aid workers but those on the receiving end.

"Nothing is more dangerous than trying to deliver aid to people who desperately need it in a situation where they may be targeted," he added.

Laerke was unable to give a timeline on when those assurances will be satisfied, saying only that there needs to be clear evidence that fighters on the ground "actually, physically cease" their hostilities.

"We need to make sure that there is compliance with this cessation of hostilities which — and I stress — is not a U.N. deal," he added. "It is a deal which has been done by the United States and the Russians, the details of which we are not privy to."

Russia's Foreign Minister Sergey Lavrvov, though, said Tuesday that Moscow wants the text of the deal made public but the U.S. is opposed.

"We've got nothing to hide," he told reporters.

The Associated Press contributed.