Aid trucks arrived in Moadamiyet al-Sham in southern Syria on Monday, taking advantage of a shaky ceasefire in the war-torn country that is now in its third day.
The ceasefire, backed by Russia and the US, went into effect across large parts of Syria at 2200 GMT Friday after major rebel factions, President Bashar al-Assad's government and the largest Kurdish militia agreed to adhere to a "cessation of hostilities."
It excludes fighting against Islamic State and other UN-designated terrorist organizations.
The ceasefire was largely holding, despite accusations of violations from the opposition that Russia and the Syrian government targeted rebel-held areas.
The UN Office for the Coordination of Humanitarian Affairs (OCHA) in Syria said that some 50 UN trucks carrying "non-food aid" entered the town of Moadamiyet al-Sham.
The UN said it will deliver humanitarian aid to hard-hit areas of Syria over the next five days to provide humanitarian assistance to about 154,000 people.
If the warring parties allow, the UN said it was ready to deliver assistance to 1.7 million additional people in the coming months. Some half a million people are in areas under siege, and the UN called for immediate access to all parts of the country.
The UN World Food Programme announced that it had received enough international donations to fully resume its food aid to 1.8 million Syrian refugees in the region, as well as to 4.5 million people inside Syria.
The opposition's allegations of ceasefire violations by the government and Russia were largely centred around Idlib, Hama and western Aleppo provinces. In those areas, there are both rebel groups who are part of the ceasefire and al-Qaeda's al-Nusra Front, which is excluded.
There had been concerns before the ceasefire went into effect over the weekend that the blurry lines between al-Nusra and the rebels could endanger the ceasefire. The extremists are fighting alongside other factions against President Bashar al-Assad.
The Syrian Observatory for Human Rights said that at least seven people, including four children, were killed late Sunday when unidentified planes bombed rebel-held areas in the western sector of Aleppo.
The Britain-based Observatory added that rebels also shelled regime forces in the city of Aleppo.
There were additional reports of fighting in other areas, including the area outside Damascus, in the Ghouta region. Rebels said they viewed this is a violation of the truce as there is only a small al-Nusra presence there and the main forces are part of the ceasefire.
Asaad al-Zoubi, a key member of the opposition's negotiating delegation, told broadcaster al-Arabiya al-Hadath that "the truce has collapsed before it even started," accusing the government of taking steps which would annul the deal.
The International Syria Support Group's Ceasefire Task Force was meeting in Geneva on Monday afternoon with French officials that have complained about alleged attacks against rebels in Syria.
UN Secretary General Ban Ki-moon acknowledged that there had been some violence despite the ceasefire. He said the international community was working "to make sure that this does not spread any further and this cessation of hostilities can continue."
The Kremlin's spokesman Dmitry Peskov cautioned against being too hasty in announcing violations of the ceasefire, saying "the situation is volatile."
Russia and the US have set up a mechanism to exchange information on the locations of groups who are taking part in the ceasefire, which does not apply to UN-designated terrorist organizations Islamic State and al-Nusra Front.
Russia, meanwhile, warned that Turkish military actions at the Syrian border had the potential to destroy the ceasefire, highlighting tensions between Moscow and Ankara.
Turkey downed a Russian jet last year along the Turkish-Syrian border, two months after Moscow began an aerial campaign in Syria to support al-Assad.
"Considering how Turkey might act, this is an extremely dangerous situation. ... It could be a ticking time bomb in the implementation of the Syrian ceasefire," Deputy Foreign Minister Sergei Ryabov said in comments carried by state news agency TASS.
Turkey has confirmed it fired at Islamic State positions in north Aleppo. Turkey is a staunch supporter of rebels, especially the ethnic Turkmen militias in the mountainous regions of the province.