Warplanes from a Saudi-led coalition hit the Houthi-controlled Red Sea port of Hodeida on Tuesday, destroying cranes and warehouses in the main entry point for aid supplies to Yemen's north.
Rival factions also battled further south overnight in Yemen's third city, Taiz, Arab television stations reported, as local militias opposed to the Houthis attempted to consolidate recent advances on it.
The human rights group Amnesty International meanwhile said the Saudi-led air campaign had left a "bloody trail of civilian death" which could amount to war crimes.
An Amnesty report said it had investigated eight coalition air strikes in Yemen that killed 141 civilians, including children.
Evidence revealed a pattern of strikes against populated areas, in most of which no military target could be located nearby, it said.
Coalition officials could not immediately be reached for comment on the Amnesty report but has previously denied targeting civilians. Amnesty also said it had investigated 30 attacks in Aden and Taiz by the Houthis that killed 68 civilians and also may amount to war crimes.
The Iranian-allied Houthis seized Yemen's capital Sanaa last September in what they called a revolution against a corrupt government, then took over much of the country.
The Saudi-backed President Abd-Rabbu Mansour Hadi fled to the southern port of Aden, then escaped to Riyadh in March. Gulf Arab countries led by Saudi Arabia intervened in the conflict to push back what they see as spreading Iranian influence in their backyard.
Loyalist forces, backed by Gulf Arab planes, weapons and training, have been on the offensive since breaking out of Aden last month, claiming a string of gains against the Houthis.
The war has killed more than 4,300 people, many of them civilians, and spread disease and hunger in one of the Arab world's poorest states.
Hodeida, lying about 150 km (95 miles) due west of Sanaa, has become a focal point of Yemen's humanitarian crisis, which the International Committee of the Red Cross said last week was critical.
Officials said the latest raids destroyed the port's four cranes and also hit warehouses, bringing work to a halt. There was no information on what was in the warehouses.
Aid groups have previously complained that a coalition naval blockade has stopped relief supplies entering Yemen. The coalition, in which the United Arab Emirates also plays a big military role, has accused the Houthis of commandeering aid shipments for war use.
Anti-Houthi groups have pushed the northern militia out of some southern provinces since late July, but while that has allowed aid to reach Aden in the south, the humanitarian crisis elsewhere remains critical.
Besides advancing from the south, coalition-backed forces are also fighting the Houthis and Saleh's troops on a second front around Marib, northeast of Sanaa.
As well as retaining a foothold in Taiz, the Houthis and allied army units loyal to former president Ali Abdullah Saleh control the northern highlands and Red Sea coastal plain as far south as Ibb, where coalition-backed forces advanced last week.
Ibb is about 50 km (30 miles) north of Taiz and 200 km southeast of Hodeida.
Saudi Arabia fears a victory for the Houthis would be used by its main regional foe Iran to encircle Gulf states and undermine their security.