An aid convoy has reached a besieged suburb of the Syrian capital Damascus, as the UN takes advantage of a partial truce brokered by the US and Russia.
The UN and its partners are stepping up deliveries of food, water and medicine, and plan to reach more than 150,000 people over the next five days.
They hope to help 1.7 million in hard-to-reach areas by the end of March.
Earlier, the UN's secretary general said the cessation of hostilities had held "by and large" since Saturday.
Ban Ki-moon also said a taskforce monitoring compliance, co-chaired by the US and Russia, would meet for the first time to evaluate alleged violations.
France has expressed concern about reports of air strikes by Syrian government and Russian aircraft on areas controlled by mainstream rebel forces.
Russia has said that it is only targeting UN-designated jihadist terrorist organisations - including the so-called Islamic State (IS) and the al-Nusra Front, which is part of a major rebel alliance - in line with the terms of the cessation of hostilities.
Meanwhile US Defence Secretary Ashton Carter stressed that the US-led coalition would also continue to fight IS during the truce.
"Let me make it crystal clear," he told a news conference in Washington. "There is no cessation of hostilities in the counter-ISIL [IS] campaign. Operations continue unabated."
The relative calm on the ground around the capital Damascus allowed 20 aid lorries carrying blankets and hygiene supplies to enter the besieged rebel-held suburb of Muadhamiya on Monday afternoon, Syrian Arab Red Crescent officials said.
"The mission is ongoing to deliver another 21 trucks by midnight," said Muhannad al-Assadi of the Red Crescent.
He said he had seen a change in the residents of Damascus since aid deliveries started.
"They told me they are very pleased with the items we're delivering to them. I saw women walking in the street, children playing in the street, a lot of children coming out of their schools with their teachers. I saw children eat. They're getting well because they are eating. Yes I saw some difference," he said.
On Wednesday, the UN and its partners plan to deliver aid to the rebel-held towns of Madaya and Zabadani, in the mountains north-west of Damascus, and the government-controlled towns of Foah and Kefraya, in the northern province of Idlib.
Against the odds - by Mark Lowen, BBC News, Gaziantep, near the Turkey/Syria border
This is now a crucial window of opportunity for the UN to get food and aid to the besieged. The truce has, in general, remained intact despite both the Western-backed opposition and regime sides complaining of dozens of violations over the weekend, including air strikes around Aleppo.
But it is unclear whether the target was the al-Qaeda-linked Nusra front, which would not constitute a ceasefire breach since it and the so-called Islamic State are not included in the deal.
A rebel spokesman talked of violations "here and there" but also of a situation much better than before. Moscow also complained of incidents but said on the whole, the ceasefire was being implemented.
That it has largely held for the weekend has defied expectations but there is still a lot of scepticism that it can continue for the full two weeks.
They are also expected to attempt another air-drop over the eastern city of Deir al-Zour, where 200,000 people in government-held areas are under siege by IS.
High winds and parachute failures meant that pallets carrying 21 tonnes of food dropped last week either missed their target, went missing or were damaged.
The UN says more than 450,000 Syrians are trapped in 15 besieged towns and villages, while 4.1 million others are living in hard-to-reach areas.
The UN's secretary general meanwhile told reporters in Geneva that "by and large the cessation of hostilities is holding, even though we have experienced some incidents".
The taskforce monitoring the truce is "now trying to make sure that this does not spread any further and that this cessation of hostilities can continue", he added.
Mr Ban also confirmed receiving a letter from the main opposition umbrella group, the High Negotiations Committee (HNC), in which it urged the UN to help "specify the territory covered by the truce to prevent hostilities in the designated inclusion zones".
The HNC's general co-ordinator, Riad Hijab, wrote that since Saturday there had been seven barrel-bomb attacks, 24 cases of artillery shelling and five cases of ground attacks by government forces, resulting in a large number of civilian deaths. Russian warplanes had meanwhile carried out 26 air strikes on territory held by rebel forces abiding by the truce on Sunday alone, he added.
Mr Hijab warned that continued violations would jeopardise the resumption of UN-brokered talks aimed at finding a political solution to the five-year conflict.
State media said armed groups had fired dozens of mortar rounds at government forces in Latakia province on Sunday, though rebels in the area denied the reports.
Syria's civil war
Why is there a war in Syria?
Anti-government protests developed into a civil war that, four years on, has ground to a stalemate, with the Assad government, Islamic State, an array of Syrian rebels and Kurdish fighters all holding territory.
What's the human cost?
More than 250,000 Syrians have been killed and a million injured. Some 11 million others have been forced from their homes, of whom four million have fled abroad - including growing numbers who are making the dangerous journey to Europe.
How has the world reacted?
Iran, Russia and Lebanon's Hezbollah movement are supporting the Alawite-led Assad government, while Turkey, Saudi Arabia and Qatar back the more moderate Sunni-dominated opposition, along with the US, UK and France. Hezbollah and Iran have pro-Assad forces on the ground, while Russia and a Western-led coalition are carrying out air strikes.