Air strikes hit the Shiite Huthi stronghold of Saada in Yemen's north, as well as other rebel positions south of the capital Sanaa and in the southern province of Lahj, residents said.
There were no immediate reports of any casualties.
The UN-proposed humanitarian truce technically went into effect at 2059 GMT Friday and is supposed to run until July 17, the last day of the holy Muslim fasting month of Ramadan.
But the ceasefire, much needed to rush food supplies to a population threatened by famine, has been flouted by strikes conducted by the Saudi-led coalition and fighting on the ground.
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More than a week ago the United Nations declared Yemen a level-3 humanitarian emergency, the highest on its scale, with nearly half the country facing a food crisis.
More than 21.1 million people -- over 80 per cent of Yemen's population -- need aid, with 13 million facing food shortages, while access to water has become difficult for 9.4 million people.