Air strikes by a coalition of Arab nations on Saada city in Yemen are in breach of international law, despite calls for civilians to leave the area, the UN Humanitarian Coordinator for Yemen said.

"The indiscriminate bombing of populated areas, with or without prior warning, is in contravention of international humanitarian law," Johannes van der Klaauw said in a statement on Saturday.

"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," van der Klaauw said.

The coalition said on Saturday it had hit Yemen with 130 air strikes over the previous 24 hours.

The coalition 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.

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.

Houthi leadership targeted

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.

Missiles also pounded rebel chief Abdul Malik al-Houthi's hometown of Marran, and nearby Baqim, Al-Masirah television reported.

Al Jazeera's Mohamed Vall, reporting from Riyadh, said the coalition's targeting of individual Houthi leaders was "a new tactic".

"They even stated previously they were after the organisation, not the individuals," Vall said. "They want to tell the Houthis that now there are no targets that are off the table."

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.

Fears of a proxy war

The Saudis and nine other Arab countries, backed by the United States, Britain and France, began airstrikes in March after Houthi rebels captured Sanaa and forced President Abd-Rabbu Mansour Hadi to flee.

The Houthis are mainly drawn from the Zaydi sect of Shia Islam that predominates in Yemen's northern highlands. The armed group, believed to be aided by forces loyal to former president Saleh, made string of military gains over the past year, emerging as a major political force in Yemen.

Riyadh fears the Houthis will act as a proxy for their main regional rival, Shia Iran, to undermine Saudi security. Iran has denied it is funding, arming or training the Houthis.