The fragile truce was declared on April 11, 10 days before Huthi Shiite rebels and the government of President Abedrabbo Mansour Hadi began UN-brokered peace talks in Kuwait.
"The ceasefire has provided us an opportunity to get to areas we haven't been able to get before," the UN humanitarian chief in the country, Jamie McGoldrick, told AFP.
He said that while no part of Yemen had been besieged, a number of areas have proved extremely hard to reach since the conflict escalated in March last year, when a Saudi-led Arab coalition began air strikes in support of Yemeni forces resisting the Huthis.
The UN was trying to assess the needs nationwide during the lull in violence, he added.
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Fighting has also continued despite the truce.
McGoldrick called Yemen's war "an invisible crisis", voicing regret at the lack of global concern given to the country, for instance compared to Syria.
"The scale of the emergency is tremendous. The scale of the need is massive and the depth of the crisis is immeasurable," he told reporters.
Government services and the health system were hardly functioning before the conflict and "the war has all but broken them completely," McGoldrick said.