The coalition of Arab states had called on civilians to evacuate Saada, the city in northern Yemen where support for Houthi rebels is strongest, before the bombing but it was unclear how they could leave.
"The indiscriminate bombing of populated areas, with or without prior warning, is in contravention of international humanitarian law," the U.N. Humanitarian Coordinator for Yemen Johannes van der Klaauw said in a statement later in the day.
"Many civilians are effectively trapped in Saada as they are unable to access transport because of the fuel shortage. The targeting of an entire governorate will put countless civilians at risk."
A coalition spokesman said the latest wave of aerial bombing, on about 100 locations, was in response to the shelling of Saudi border areas by Houthi forces this week.
The air strikes targeted bases of Houthi leaders across Saada and Hajja provinces, said Brigadier General Ahmed Asseri, as well as hitting tanks and other military vehicles.
Other strikes targeted Sanaa airport's runway, a Yemeni official there said, and Houthi targets in the al-Sadda district of Ibb in central Yemen, residents there said.
In the southern port city of Aden, clashes continued on Friday and Saturday in the central Crater, Khor Maksar and Mualla districts as the Houthis and forces loyal to former president Ali Abdullah Saleh shelled local militias trying to oust them from the city.
However, the Houthis were pushed back from parts of Dar al-Saad in the city's north into Lahj Province, local militias said, and faced fighting in al-Dhala Province.
In Shabwa province, east of Aden, four men including a suspected al Qaeda leader were killed in a drone strike, local officials said.
The coalition has bombarded the Houthis and army units loyal to Saleh since March 26, but eased back on the strikes in late April and on Friday offered a five-day truce starting on May 12 if other parties agreed.
The Saudis and nine other Arab countries, backed by the United States, Britain and France, hoped to force the Houthis back to their northern heartland and restore the exiled government of President Abd-Rabbu Mansour Hadi, who is in Riyadh.
The Houthis are mainly drawn from the Zaydi sect of Shi'ite Islam that predominates in Yemen's northern highlands. They took advantage of political chaos to seize Sanaa and then advance further south over the past year, aided by Saleh.
Riyadh fears the Houthis will act as a proxy for their main regional rival, Shi'ite Iran, to undermine Saudi security, and that their advance into Sunni regions will add a sectarian edge to the civil war, strengthening an al Qaeda group in Yemen.
Iran and the Houthis deny funding, arming or training is coming from Tehran, and analysts say the rebel group is unlikely to become an all-out proxy for the Islamic Republic in the mould of Lebanon's Hezbollah.
Iran's President Hassan Rouhani said on Saturday the Saudi-led campaign was the work of an "inexperienced" government that did not understand the region's politics.
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