Houthi forces fought battles street by street with local militia in the old centre of Aden on Wednesday, as the first boatloads of emergency medical aid arrived to the south Yemeni port city which aid 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.
By mid-afternoon residents of the central Crater neighbourhood said the Houthi push, backed by tanks and armoured vehicles, had been at least partially repelled, and that Houthi fighters had been cleared from some northern neighbourhoods.
Iran, which denies arming the Houthis, has condemned the Saudi-led offensive. It 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 Muslim Shi'ite fighters, who control the capital Sanaa. Their assault prompted Tehran's regional rival Saudi Arabia and its allies to launch air strikes against the Houthis.
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 the group had delivered to Aden since the fighting escalated.
The ICRC said a surgical team also reached Aden on Wednesday by boat, and was heading to a hospital in the city of 1 million people.
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 by fighting on the ground and by the air strikes.
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.
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.
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 request from Saudi Arabia for it to join the military operation. Prime Minister Nawaz Sharif has said he will defend Saudi Arabia's "territorial integrity" but not 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.
Alawi previously said Oman was ready to help mediate between the two sides but he did not believe they 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 U.S. 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.