Tribesmen loyal to exiled President Abd Rabbuh Mansour Hadi advanced on Shiite Houthi militia strongholds in the north of Yemen as Saudi-led coalition warplanes carried out fresh raids on Houthi positions in the south, tribal and army sources said on Thursday.

Coalition warplanes twice bombed a Houthi gathering on a hill overlooking Taiz, from which the militiamen have been attacking residential neighbourhoods of Yemen's third city, a local government official told AFP.

He said four civilians were killed and six wounded in overnight mortar fire by anti-Hadi forces - Houthis and renegade troops loyal to former president Ali Abdullah Saleh.

The warplanes also struck Houthi militia positions in nearby Daleh as well launching three dawn raids on Al-Anad air base in the southern province of Lahj, military sources said.

Arms depots of pro-Saleh forces at Dhamar and Baida provinces in central Yemen also came under air attack several times, residents said.

In the main southern city Aden, witnesses reported sporadic fighting on the northern outskirts between Houthi militia and forces loyal to Hadi.

An anti-Hadi leader, Nabil al-Hashidi, was killed late on Wednesday when pro-Hadi fighters stormed his hideout in Aden, tribal sources said, accusing him of "spying" for Iran.

In the Houthi-held northern province of Jawf, pro-Hadi tribesmen seized control of Yatamah, a town on the border with Saudi Arabia, tribal sources said.

The loyalists operated under the cover of coalition airstrikes as they later advanced towards a town in the Houthi stronghold province of Saada, the sources said.

These advances come as the pro-Hadi forces chief of staff, General Mohammed Ali al-Maqdishi, arrived in the Ramlat al-Sabiin area near the Saudi border to inspect his troops, tribal sources said.

The Saudi-led coalition has waged an air campaign against the Houthi militia since 26 March in an effort to restore the authority of Hadi, who has fled to Riyadh with members of his government.

A United Nations conference to relaunch political talks on Yemen will open in Geneva next week, a UN spokesman said on Wednesday, despite uncertainty over who will attend.

UN chief Ban Ki-moon said the 28 May meeting was to "restore momentum towards a Yemeni-led political transition process" after weeks of conflict that have left 1,850 dead.