The forces loyal to President Abedrabbo Mansour Hadi have been pressing to recapture parts of Yemen seized by the Shia Huthi rebels who overran the capital last September and then advanced south.
They have been backed by a Saudi-led coalition that launched air strikes in March and began a major counter-offensive in July.
Presumed US reconnaissance drone crashes in Yemen
"The military operation to liberate Taez has begun after the arrival of military reinforcements from the Arab coalition, resistance forces and the national army in the south and west of Taez province," General Ahmed al-Yafie, commander of the fourth military region, told AFP.
The region includes Taez as well as Yemen's second city Aden further south. Military officials have said the coalition sent major reinforcements before they advanced towards Taez, Yemen's third biggest city seen as a gateway to controlling the rebel-held capital.
Armoured demining vehicles and other reinforcements arrived in the Shuraija region, between the loyalist-controlled Lahj province and Taez in the southwest, said officials in the main southern city of Aden.
Sudanese forces from the strategic Al-Anad airbase in Lahj are taking part in the Taez operations, according to the sources.
Taez has seen heavy fighting in recent months between the Huthi rebels and forces of Hadi's internationally recognised government.
Loyalist forces are inside Taez, while the rebels and troops loyal to former president Ali Abdullah Saleh control the main roads leading into the city.
Along the coast, coalition troops deployed in Dhubab in a bid to advance onto the rebel-held Red sea city of Mocha, a part of Taez province, the sources said.
Pakistan and Saudi Arabia have made peace after Yemen war rift: US security expert
On Sunday, military officials spoke of major coalition reinforcements of troops and equipment arriving to Taez from Aden.
Meanwhile, three pro-government fighters and two rebels were killed in clashes an area on the border between Taez and Lahj, military officials said, adding that loyalists managed to retake a hill overlooking the area.
This month, a 400-strong Sudanese force arrived in Aden in support of loyalist forces, joining 500 who arrived last month.
Some 5,000 people have been killed in the conflict since March, more than half of them civilians, according to the United Nations. A new round of UN-brokered Yemen talks to end the conflict is expected to kick off in Geneva this month.
COMMENTS
Comments are moderated and generally will be posted if they are on-topic and not abusive.
For more information, please see our Comments FAQ