Houthi forces fought street-by-street battles with local militia in the old centre of Aden on Wednesday, as the first boatloads of emergency medical aid reached the south Yemeni port city, which relief workers say faces a humanitarian catastrophe.
Residents saw a dozen bodies strewn on the streets and said several buildings were burnt or demolished by rocket fire. Mosques broadcast appeals for jihad against the Houthis, Iran-allied fighters who have taken over large areas of Yemen.
The Houthi attack in the central Crater neighbourhood, backed by tanks and armoured vehicles, was at least partially repelled, residents said, and Houthi gunmen had also been driven from some northern neighbourhoods.
Iran, which denies arming the Houthis, has condemned the Saudi-led offensive. Tehran sent two warships to the Gulf of Aden on Wednesday, saying they would protect Iranian shipping.
Aden has been the target of a three-week-old assault by the Shi'ite Muslim fighters, who control the capital Sanaa. Their campaign prompted Tehran's rival Saudi Arabia and its allies to launch air strikes against the Houthis.
Saudi Arabia's leading role against the Houthis has turned Yemen into the latest theatre of a regional proxy conflict between the Gulf's leading Sunni Muslim and Shi'ite Muslim powers - a struggle also playing out in Syria, Lebanon and Iraq.
The fighting has had a devastating impact on parts of Aden. Scores of people have been killed, water and electricity have been cut off in central neighbourhoods, and hospitals have struggled to cope with the casualties.
"It's nearly catastrophic," said the International Committee of the Red Cross spokeswoman in Yemen, Marie Claire Feghali.
"Shops are closed, so people cannot get food, they cannot get water. There are still dead bodies in the street. Hospitals are extremely exhausted."
A boat carrying 2.5 tonnes of medicine docked in Aden on Wednesday, the medical aid group Medecins Sans Frontieres said. MSF said it was the first shipment it had delivered to the city since the fighting escalated.
The ICRC said a surgical team also arrived by boat in Aden, which has a population of one million.
The World Health Organisation says at least 643 people have been killed in the conflict and more than 2,200 wounded. Tens of thousands of families have been displaced.
Iran has called for an immediate halt to the air strikes by the Saudi-led coalition which includes four other Gulf Arab states, and appealed for dialogue.
Its deputy foreign minister said Yemeni factions should form a national unity government. "We in the Islamic Republic of Iran are undertaking all good initiatives and efforts that help in reaching this political solution," Morteza Sarmadi said.
Iran's Alborz destroyer and Bushehr support vessel would patrol the Gulf of Aden and Red Sea to protect Iranian shipping from piracy, navy chief Rear Admiral Habibollah Sayyari said.
United Arab Emirates Foreign Minister Sheikh Abdullah bin Zayed said Iran was meddling in the conflict and elsewhere in the region. "There is systematic action that has been going for years in the idea of exporting the (Iranian) revolution," he said in Abu Dhabi after talks with Yemen's foreign minister.
A military spokesman for the Saudi-led coalition said Iran had the right to send ships to international waters as long as they had no aggressive intent.
But he repeated accusations that Iranians in Yemen had supported, trained and equipped the Houthi fighters, adding that any of them who remained "are now in the same trench as the Houthi militias, and will face the same fate".
The United States, a major Saudi ally, said on Tuesday it was speeding up arms supplies for the offensive, and had increased intelligence sharing and planning coordination.
State media in the United Arab Emirates says Saudi Arabia has deployed 100 jets in its air campaign, alongside 30 from the UAE, 15 each from Kuwait and Bahrain, and 10 from Qatar. Sudan, Jordan, Egypt and Morocco have also supported the campaign.
Pakistan's parliament is debating a Saudi request for it to join the military operation. Prime Minister Nawaz Sharif has promised to defend Saudi Arabia's "territorial integrity", but has not said what, if any, commitments he has made.
The foreign minister of Oman, the only member of the Gulf Cooperation Council (GCC) not participating in the bombing campaign, called for a short humanitarian truce after meeting his Iranian counterpart in Muscat on Wednesday.
Yusuf bin Alawi previously said Oman was ready to help mediate between the two sides but he did not believe the combatants were ready to come to the table.
Overnight, warplanes struck al-Anad airbase, about 50 km (30 miles) north of Aden, local officials said.
The base, which once housed US military personnel involved in Washington's covert drone war against al Qaeda fighters in eastern Yemen, has been taken over by soldiers loyal to former president Ali Abdullah Saleh, who are allied to the Houthis.
There were also air strikes against Houthi positions in the town of Dhalea, further north from al-Anad.