At least 57 people have been killed as Yemeni pro-government forces gained ground around third city Taiz which has been under Houthi siege for several months, officials said.

The loyalists backed by jets from a Saudi-led military coalition took back areas in the western and southern suburbs of the city on Friday, said governor Ali al-Maamari.

They "reopened key roads that the Houthis had been blocking for nine months," said the governor, who lives in exile in Saudi Arabia.

That should allow for humanitarian and medical aid to reach about 200,000 besieged inhabitants, he said.

Loyalist military sources said clashes between pro-government forces and air strikes had killed at least 57 people on Friday, 37 of them rebels, six civilians and the rest loyalist fighters.

Earlier a source in the army's 35th brigade confirmed that loyalists had seized al-Misrakh area to the south of Taiz city after heavy fighting that led to several deaths in the past few days.

Dozens of military vehicles carried rebel fighters out of the western suburb of Taiz towards the city of Hodeida on the Red Sea, witnesses said.

The coastal city remains under the control of the insurgents and their allies, army units loyal to former president Ali Abdullah Saleh.

Taiz is located between the rebel-held capital Sanaa and the southern port city of Aden, which loyalists took back from the Houthis in July.

In November, forces loyal to President Abd Rabbuh Mansour Hadi announced a major offensive to try to break the siege on Taiz.

More than 6,100 people have died - half of them civilians - since the Saudi-led coalition launched airstrikes on Yemen in March 2015, according to the United Nations.