The cease-fire "is fragile but largely holding," envoy Ismail Ould Cheikh Ahmed said in a statement.
He urged participants "to show restraint, avoid further escalation, and strictly adhere to the 72-hour ceasefire."
The UN official is liaising with the parties in an attempt to extend the ceasefire in order "to create a conducive environment for a long-lasting peace" in Yemen.
The envoy also "reminds all parties that the terms and conditions" of the ceasefire "include commitments for the unhindered access for humanitarian supplies and personnel to all parts of Yemen."
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A Saudi-led Arab coalition intervened in Yemen in March 2015 to support the government of President Abedrabbo Mansour Hadi after Huthi rebels overran much of the impoverished country.
Five previous truce attempts failed, but cautious optimism preceded the current pause after an escalation of combat led to intensified international pressure.
Nearly 6,900 people have been killed in the conflict, more than half of them civilians, while an additional three million are displaced and millions more need food aid.