Houthi rebels and allies backed by tanks pushed into central Aden, a strategic port city and the main foothold of fighters loyal to President Abd-Rabbu Mansour Hadi, witnesses said on Wednesday.
Their relentless advance came despite a week of airstrikes by Saudi-led coalition forces. The alliance of mainly Sunni Gulf Arab states also has attacked the northern Shi'ite Houthis from the sea.
Hadi's aides expressed alarm.
"What's happening now would be a disaster for Aden and its people, if Aden falls," Reyad Yassin Abdulla told Al Jazeera television.
The Houthi movement was jubilant.
Followers of the Houthi group demonstrate in Sana'a against the Saudi-led airstrikes on Yemen, April 1, 2015.
"We can say that after a week of bombing on Yemen, the aggressors have not achieved any result," spokesman Mohammad Abdulsalam told the militia’s al-Maseera television. "... The victories in Aden today embarrass this campaign and silenced the aggressor states."
Asked for his reaction, a Saudi military spokesman said Houthi forces had been in Aden before the alliance began its campaign on March 26 and had fought daily battles against city residents.
The Houthis, with vital support from army units loyal to former president Ali Abdullah Saleh, emerged as the dominant force in Yemen after they took over the capital six months ago. They now control much of the Arabian Peninsula.
Aden residents said they saw groups of fighters carrying rocket-propelled grenades and accompanied by four tanks and three armored vehicles in the Khor Maksar district, part of a land link between central Aden and the rest of the city.
The unit met strong resistance from local militias. Residents said they saw the bodies of eight Houthi fighters on the street. Rocket fire fell on the central Crater district.
Houthi rebels reportedly have reached the center of Aden, Yemen's strategic port city.
Many people fled the area and some were trying to get on a ship leaving the port.
Earlier Wednesday, dozens of fighters were killed in clashes between Houthi fighters and their army allies on one side, and militiamen and tribesmen opposing them around Aden and elsewhere in south Yemen, witnesses and militia sources said.
Hadi left Aden Thursday for Saudi Arabia, whose stated aim is to restore him to power.
In Dhalea, 100 km (60 miles) north of Aden, airstrikes supported militiamen fighting street battles against the Houthis, who are allied with Saudi Arabia's regional foe Iran. The Houthis are backed by army units loyal to longtime ruler Saleh, who was pushed out three years ago after Arab Spring demonstrations.
Ten of the militia fighters were killed, Dhalea residents said, but Houthi forces and allied army units were being pushed back.
In the southeastern province of Shabwa, the Houthis suffered heavier losses in battles with tribesmen at a major army base. Some 35 Houthi and army fighters were killed along with 20 tribesmen.
Meanwhile, the Saudi-led air attacks continued on targets nationwide overnight.
An explosion at a dairy factory in Yemen's Hodaida port killed at least 25 workers, medical sources said, with conflicting accounts attributing the blast to an airstrike or to a rocket landing from a nearby army base.
The 26September website of Yemen's factionalized army, which mostly sides with the Houthis, said 37 workers were killed and 80 wounded at the dairy and oils factory "during the aggressive airstrikes which targeted the two factories last night."
Medical sources in the city said 25 workers at the plant had been killed at the factory, which was located near an army camp loyal to Saleh.
Residents and witnesses contacted by Reuters said the airstrikes had targeted the factory shortly after midnight Wednesday. Others said the factory was hit by rockets fired from the base, possibly as retaliation for the bombings.
Raids in north
Other airstrikes hit Houthi positions along the Saudi border in Yemen's far north, an army base in the central highlands, air defense infrastructure in the eastern Marib province, and a coast guard position near Hodaida.
A raid at a coastal defense station at Maidi port in Hajja province north of Hodaida killed six soldiers, workers there said, while further strikes hit an army camp in Sana’a and a government facility in Sa’adeh in the north of Yemen.
UNICEF said at least 62 children had been killed and 30 wounded in the violence over the past week. The United Nations said an attack on a refugee camp in northern Yemen, which medics blamed on an airstrike, broke international law.
Not including Wednesday's toll, 103 civilians and fighters had been killed in the city since clashes began last Tuesday, according to the Field Medical Organization, a nongovernmental group based in Aden.