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Fighters allied to Yemen's exiled president seized about 10 southern villages from Houthi forces on Tuesday, pursuing their offensive a day after capturing the country's biggest air base, residents and anti-Houthi sources said.

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

Local militias, known as the Popular Resistance, and army units trained and equipped by Gulf Arab countries have made advances against the Iranian-allied Houthis, as well as allied Yemeni army units loyal to ex-president Ali Abdullah Saleh, in recent weeks.

Yemeni military sources said that 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 air strikes, the fighters, many of whom are secessionists fighting under the flag of the former South Yemen, drove the Houthi-Saleh forces from the port city of Aden last month and then pushed northward and recaptured the al-Anad air base from the Houthi-Saleh forces on 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 Lahj," a commander in the anti-Houthi forces told Reuters.

Militia sources said a thousand Yemeni fighters trained in Saudi Arabia and the UAE arrived in Aden on Monday.

Yemen's Arab neighbors intervened in the country in March to halt the advance of the Houthis - predominantly Zaydi Shia 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 arch rival, Shia Iran, and aim to restore Hadi to power in Sanaa.

The Houthis have been pounded with hundreds of air strikes 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.

In a significant move for desperately-needed humanitarian supplies, two officials from the exiled government told Reuters it would now require all aid ships to dock at the port of Aden, now controlled by anti-Houthi forces.

Arab ships have been imposing a near-blockade on Yemen to bar arms transfers to the group but had permitted food, fuel and aid to regularly dock at ports held by the militia to relieve civilians in the hinterlands suffering from shortages.

The U.N. envoy to Yemen, Ismail Ould Cheikh Ahmed, renewed his call on Tuesday for an immediate ceasefire 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 ceasefire 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," he said.