A ceasefire in Syria was largely holding on Tuesday as the death toll from the country's five-year conflict exceeded 300,000.

"Today calm appears to prevail" following sporadic violations of the ceasefire in the first hours after it went into effect Monday evening, UN Syria envoy Staffan de Mistura told reporters in Geneva.

He noted, however, that it was up to the United States and Russia to make a first formal assessment 48 hours after the start of the truce that was brokered by these two powers.

Activists and residents reported a feeling of calm returning to Aleppo, Syria's former commercial hub which has become a key battleground in the multi-sided conflict.

"The night was quiet for the first time in months. We have not heard any plane flying or strikes," activist Abdel-Mouneim Juneid, based in a rebel district in Aleppo, said.

"But, people inside Aleppo are cautiously moving inside the city because we do not trust the regime," he told dpa.

Alaa, a resident of a regime-controlled district in Aleppo's al-Zahra neighbourhood, sounded optimistic about the truce. 

"Last night, it was the first night we slept without hearing sounds of bombs and shells," he told dpa by phone.

"This ceasefire raises our hopes."

However, no UN humanitarian aid trucks were driving to besieged eastern Aleppo as the government and local authorities have yet to guarantee unhindered access, de Mistura said.

"The UN is ready. There is a little homework still to be done by others," he said.

Ceasefire violations were reported outside Aleppo, as well as in the central province of Hama, according to the Britain-based Syrian Observatory for Human Rights.

At least two people were killed when unidentified warplanes struck suspected Islamic State fighters in the central province of Homs, the monitor added.

Meanwhile, Russia accused US-backed rebel groups of having violated the truce 23 times by Tuesday morning.

"Syrian government troops have stopped all fire everywhere, except for areas where Islamic State and al-Nusra Front terrorists still operate," General Viktor Poznikhir said in comments carried by Russian state news agency TASS.

"Regrettably, the same cannot be said about armed groups of the US-controlled moderate opposition," Poznikhir added.

In Washington, US State Department spokesman Mark Toner said the US had seen "some reports of sporadic violence, but thus far, the arrangement as a whole seems to be holding and the violence ... is lower in comparison to previous days and weeks.

"But as I said, we anticipated an uneven start to the cessation of hostilities."

The ceasefire excludes Islamic State extremists and al-Qaeda-affiliated militants.

Russia is a key ally of Syrian President Bashar al-Assad.

On Friday, the US and Russia sealed an ambitious agreement on Syria that also envisages unprecedented military coordination against militant groups there.

Russia has set up a "mobile observation point" and operative military groups in Aleppo to monitor the ceasefire, TASS news agency reported.

A Russian humanitarian convoy used the vital Castello Road on Tuesday to deliver supplies, the report said.

Syria said it would not allow aid from its vociferous neighbour, Turkey.

"The Arab Republic of Syria announces its rejection of allowing entry of such [aid] from any side, especially from the Turkish regime, without coordination with the Syrian government and the United Nations," an unnamed source at the Foreign Ministry told state news agency SANA.

Turkey supports opposition rebels fighting to oust al-Assad and has repeatedly called on him to step down to end the conflict, which has left more than 300,000 dead.

The monitoring group said on Tuesday it has documented the deaths of 301,781 people since Syria's crisis started in March 2011.

The toll included 86,692 civilians, 10,009 of which were children.

The current truce is the second this year to be brokered by Washington and Moscow. The first, which went into effect in February, failed to hold for long.

UN-brokered peace talks involving the Syrian government have been stalled since April when the previous truce crumbled, but de Mistura said the new ceasefire and increased aid could revive them in the coming weeks.

"In other words, no bombs and more trucks," he said.

Russia and the US could then set up their planned Joint Implementation Centre to separate terrorists from rebel fighters, he said.

After that, global and regional powers could gather on the sidelines of the UN General Assembly in New York and the UN Security Council would hold a meeting on Syria on September 21, according to de Mistura.

This could be followed by invitations being issued to the conflict parties for new peace talks, he said.

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