"Today, I announce the start of military operations to retake the remaining territory of the republic and extend the authority of the state and its institutions across the entire national territory," Yemeni President Rashad al-Alimi said in a televised address.
The conflict between the Iran-aligned Houthis and forces aligned with the Saudi-backed government mostly stopped after a 2022 truce but fighting has broken out again in a spillover from the wider war between Iran and the United States.
Saudi Arabia entered neighbouring Yemen's civil war in 2015 at the head of an Arab coalition in support of the internationally recognised government, which the Houthis had driven out of the capital Sanaa.
Hostilities resumed this year after the Houthi movement declared a naval blockade on Saudi Arabia in July and began attacking Saudi territory and shipping, while Houthi forces also made major territorial gains along Yemen's Red Sea coast.
Saudi Arabia has responded with air strikes in Yemen and backed government forces fighting to repel Houthi fighers.
Regional officials had previously told Reuters that Saudi Arabia and its Yemeni allies were planning to launch an assault on the Houthi rebels, with the Saudis leading aerial combat and Yemeni forces fighting on the ground.
Despite the preparations for a counteroffensive, the Houthi rebels have so far maintained the initiative on the ground, weeks after seizing the strategic Bab el-Mandeb Strait in a lightning offensive down the coast during which government forces collapsed.
Bab el-Mandeb is the southern exit passage of the Red Sea, a key shipping route between the Mediterranean and Asia that has assumed even more importance this year as an alternative to the Strait of Hormuz, which has been effectively closed by the Iran conflict.
In the last 48 hours, the Houthi rebels have made gains against government and local forces on strategic hilltops near Yemen's Taiz, military sources said, advancing on the last road between the city and Aden, where the Saudi-backed Yemeni government is based.
