An aid convoy has reached one of several besieged towns in Syria, 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.
The relative calm on the ground around the capital Damascus allowed 10 aid lorries carrying blankets and hygiene supplies to entered the suburb of Muadhamiya on Monday afternoon, Syrian Arab Red Crescent officials 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.
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 under siege, while 4.1 million others are living in hard-to-reach areas.
“Some of these people have not been receiving assistance for months or even up to a year in some cases, so it’s really, really important that we get food in and other kinds of assistance,” Greg Barrow of the World Food Programme told the BBC.
“We’re very concerned about the nutritional status of people living in those areas that have really been sealed off from the outside world,” he added.
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, 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.