ADEN, Yemen: Fighters loyal to Yemen’s deposed president seized about 10 southern villages from Houthi forces Tuesday, maintaining momentum in their offensive a day after capturing the country’s biggest air base, residents and loyalist sources said.

Clashes took place across the southern province of Lahej, most of which is now back in the hands of the Saudi-backed loyalist forces.

Militias siding with President Abed Rabbou Mansour Hadi, who fled Yemen for Saudi Arabia in March, and army units trained and equipped by Gulf Arab countries have made advances against the Iranian-allied Houthis in recent weeks.

Yemeni military sources said the United Arab Emirates had sent in dozens of tanks and heavy artillery pieces to the fighters in recent days, though a spokesman for the Arab coalition denied media reports that it had sent ground troops.

Boosted by Saudi-led airstrikes, they drove the Houthis from the port city of Aden last month then pushed northward and recaptured the al-Anad air base from Houthi forces Monday after besieging it for days.

“The next step for the popular resistance and army forces after liberating Aden is the clearing of the provinces of Abyan and Lahej,” a commander in the anti-Houthi forces told Reuters.

Meanwhile, at least 15 pro-government fighters were killed by two land mine explosions in their push north toward the Houthi-controlled Al-Houta, the capital of Lahej province, both pro-government militia and Houthi officials said.

In Taiz, Yemen’s third largest city, ground fighting raged for hours, killing at least seven and wounding 47 fighters, security officials from both sides said. Two civilians were also killed, medical officials added.

All officials spoke on condition of anonymity because they are not authorized to speak to reporters.

Yemen’s Arab neighbors intervened in the country in March to halt the advance of the Houthis – Shiite Muslims from the north whose fighters seized the capital Sanaa in September and took over most of the country. The Sunni Muslim Gulf states say the Houthis are a proxy for their archrival, Shiite Iran, and aim to restore Hadi to power in Sanaa.

The Houthis have been pounded with hundreds of airstrikes for more than four months and the raids and other warfare has killed more than 4,000 people. Disease, hunger and water shortages have also contributed to a humanitarian crisis.The U.N. envoy to Yemen, Ismail Ould Cheikh Ahmed, renewed his call Tuesday for an immediate cease-fire in the conflict, based on a plan involving Houthi withdrawals from main cities to pave the way for the exiled government’s return.

“There must be a withdrawal, a cease-fire and an agreement on them both,” Ould Cheikh Ahmed told Egyptian TV channel CBC. “The government [must] return gradually to perform its duties in infrastructure and services ... it must return to Sanaa and to the big cities, that’s essential.”

In Geneva, the U.N. refugee agency said 100,000 people have fled the fighting in Yemen over the past four months. UNHCR spokesman Adrian Edwards told reporters that the agency only has one-fifth of the funds needed to meet this outflow.

“With funding also low for operations inside Yemen, UNHCR is concerned that delivery of assistance there, as well as to refugees fleeing the country, will be at risk without additional funding soon,” Edwards said.

And Doctors Without Borders (MSF) said that health services in Yemen were “nearing collapse,” with pregnant women dying for lack of transport and hospitals under fire from snipers. Shortages of food, medicine and health workers are acute, said Thierry Goffeau, MSF’s coordinator in Aden.

“The population is faced with food shortages, the health system is in a state of collapse. I have never seen such a level of violence,” he told a news conference in Paris following a 10-week mission to Yemen.

Transport has been totally disrupted by the near-daily air raids, meaning that “women needing a cesarean die because they cannot get to a hospital in time,” said Laurent Sury, MSF’s head of emergencies.