UN and its allies are working up to provide food, water and medicine to people trapped in besieged capital of Syria, Damascus.
Taking advantage of the truce between the parties involved, they hope to help 1.7 million in hard-to-reach areas by the end of March.
UN Secretary General 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.”