Rebel forces and loyalist fighters were locked in fierce fighting in Yemen on Monday as hopes of a political solution faded after the United Nations postponed peace talks.
Clashes raged in the country's third city Taez, sowing panic after Shia Houthi rebels bombarded several districts with rocket and tank fire.
"There's a real massacre going on in Taez, the city that spearheaded the revolt" against former president Ali Abdullah Saleh who now supports the Houthis, one resident said.
"Saleh has aligned himself with the rebels to take revenge," Bassam al-Qadhi added.
Saleh quit in early 2012 after bloody year-long protests against his rule, and was replaced by Abd-Rabbou Mansour Hadi who has taken refuge along with his government in neighbouring Saudi Arabia.
Monday's clashes in Taez came just four days before a new UN effort was to have been held, aiming to resolve the conflict in the impoverished Arabian Peninsula nation.
But there had been growing uncertainty over which of the warring parties would attend the talks slated for Thursday in Geneva.
"I can confirm that the meeting has been postponed," a UN official told AFP on Sunday, without providing further immediate explanation.
The postponement is another blow to UN efforts to broker peace in a country where nearly 2,000 people have been killed since March.
Hadi had laid out his government's demands in a pre-talks letter to UN Secretary General Ban Ki-moon, insisting that the rebels must withdraw from territory they have seized.
He reiterated his position on Sunday during talks in Riyadh with Ban's special envoy, Ismail Ould Cheikh Ahmed.
In a bid to restore Hadi's authority, Saudi Arabia has led an air campaign against the Houthis since March 26.
Coalition aircraft bombarded targets across the country at the weekend, including the airport in second city Aden and its northern suburbs, officials said.
On Monday, residents and an official in Taez said fierce fighting had killed and wounded dozens of people.
The clashes, which erupted on Sunday and raged overnight, have killed at least 30 Houthi rebels and allied forces, the official told AFP.
He said a provisional toll showed at least five Hadi loyalists were also killed.
For a second consecutive day, rebels and their allies targeted several Taez neighbourhoods with rockets and tank shells, residents said.
Medics and residents told AFP late Sunday the bombardment had killed 10 civilians and wounded 80.
Officials in neighbouring Daleh province said pro-Hadi fighters had retaken several positions from the rebels, including a military camp, in dawn fighting on Monday.
They said clashes were ongoing, especially at a now rebel-held base of the 33rd Armoured Brigade where pro-government forces have seized six tanks.
Clashes were also reported in other southern provinces on Monday, including Shabwa and Abyan as well as Aden.
The fighting followed a day of intense raids by Saudi-led coalition warplanes targeting rebel arms depots and positions, witnesses said.
More than 545,000 people have been displaced in the conflict, and although some aid trickled in last week during a five-day ceasefire, people still lack basic needs, including water, electricity and fuel.
A ship carrying 460 tonnes of Emirati humanitarian aid docked on Sunday in Aden.
The shipment, including medical and food supplies, is the second from the United Arab Emirates following the delivery of 1,200 tonnes of aid last week, said local aid coordinator Ali al-Bikri.
Another ship carrying 400 tonnes of diesel also arrived on Friday, said Bikri, who was appointed by Yemen's government in exile.
"Aden needs urgently at least 200,000 food rations for the displaced," Bikri added.
Fierce fighting has raged in Aden between the rebels and their allies and southern fighters allied with Hadi.
Hadi has demanded full implementation of UN Security Council Resolution 2216 calling on the Houthis to relinquish territory they seized and surrender weapons they took from the army and other state institutions.