(CNN) US Secretary of State John Kerry and Russian Foreign Minister Sergey Lavrov agreed to extend the ceasefire in Syria for another 48 hours, State Department spokesman Mark Toner told reporters Wednesday.

Toner acknowledged the situation hasn't been perfect.

"We've seen violations by both sides," he said, adding, "we're continuing to monitor this very closely."

The move comes as the ceasefire in the brutal civil war appears to be holding into its second day, with one monitoring group reporting no deaths in the area covered by the US-Russia agreement.

The UK-based Syrian Observatory for Human Rights on Wednesday said no fatalities have occurred in Homs, Latakia, Hama, the Damascus suburbs, Idlib and the rest of the Syrian areas included in the deal that began Monday.

The state-run Syrian Arab News Agency reported that "armed groups" breached the truce with mortar fire, citing a military source; the news agency also reported there were no deaths.

Convoys poised to deliver aid

But for hundreds of thousands of besieged Syrians, the wait for humanitarian relief may last somewhat longer.

Aid convoys are positioned at the Turkish border town of Cilvegozu, poised to enter the country and deliver food and medical aid to rebel-controlled eastern Aleppo, where the United Nations says between 250,000 and 275,000 people have been cut off from assistance since early July.

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An activist with the opposition-aligned group Aleppo Media Center said people are hopeful this initiative will work, even though past agreements have fallen apart since the conflict began in 2011.

"We are still optimistic this will happen, but no one knows," he said. "But so far it's been the best days seen since five-plus years ago. This is something promising. We shall wait and see what happens tomorrow."

Jens Laerke, a spokesman for the UN humanitarian office, said the aid convoys were awaiting assurances of safety in Syria's volatile northwest.

"The UN convoy from Turkey to eastern Aleppo is still in customs area on the border this morning because of lack of de facto assurances of safe passage by all parties," Laerke said.

"The convoy is ready and will move as soon as conditions allow."

Staffan de Mistura, the UN special envoy for Syria, said Tuesday night that there was a "need of assurances that the drivers and the convoys will be unhindered and untouched."

Syria's Foreign Ministry has said aid can only be delivered to Aleppo if it is coordinated through the government and United Nations -- especially aid coming from Turkey.

The Syrian military is ready to pull out of a key Aleppo highway -- Castello Road -- to allow safe passage of humanitarian aid into the city, according to Vladimir Savchenko of the Russian Center for Reconciliation in Syria, Russian state news agency Tass reported.

The Syrian government has not confirmed or reported this news.

The Aleppo activist said it's the first time the United States and Russia have forged an agreement that "paves the way for humanitarian aid access" and a no-military zone for people to use the highway.

No major violence

Nevertheless, the ceasefire appears to have led to a respite in violence in the five-year civil war.

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De Mistura, the UN envoy, noted a "significant drop in violence" since the ceasefire went into effect Monday.

There were still intermittent incidents, including an airstrike overnight on Khan Toman in the countryside south of Aleppo, according to an Aleppo Media Center activist.

In general, the situation remained quiet in Aleppo, with no significant violations, the activist said.

The state-run Syrian Arab News Agency said that rebel groups had breached the truce by firing seven mortars Wednesday morning in the countryside near Homs.

In recent days, images have emerged from Syria showing people taking advantage of the rare calm to celebrate the Islamic holiday of Eid al-Adha

Terror groups excluded from truce

The ceasefire deal, intended to bring humanitarian relief, calls for a halt to the violence between the Syrian regime and rebel forces.

But it does not cover militant groups considered terrorists, such as ISIS and Jabhat Fateh al-Sham , formerly known as al Nusra Front.

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

Russia's warplanes have attacked a group of ISIS militants north of Palmyra, killing hundreds of terrorists, Viktor Poznikhir, the first deputy chief of the General Staff's main operations directorate, told the media Wednesday.

The militants lost 250 men and 15 pickup trucks, Poznikhir said.

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.

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.

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.