Iran-backed Houthi rebels have captured more ground in southwestern Yemen, taking the key town of Turbah and moving towards a strategic provincial capital in a setback for Saudi-backed government forces. The town fell on Sunday, shortly before the internationally recognised government announced a major military operation against the Houthis. The latest escalation between Saudi Arabia and the Houthis has revived Yemen's civil war after a four-year ceasefire.

Turbah is on the main road linking the provincial capital of Taiz with the port city of Aden, which serves as the interim seat of the internationally recognised government, and has long been an important position for forces defending Taiz. Before taking Turbah, the Houthis had seized several villages and areas on the outskirts of Taiz, opening the way to encircle Yemen's third-largest city. In early September, the rebels also made a swift push along the country's western Red Sea coast, capturing the strategic city of Mokha and several Red Sea islands.

Those gains enabled them to control the Yemeni side of the Bab al-Mandab Strait, a key chokepoint through which 12 per cent of global trade passes. In an announcement over the weekend, the Saudi-backed government said it was mobilising troops across areas under its control. Rashad al-Alimi, head of the ruling Presidential Leadership Council, said in a televised speech that the operations were aimed at "recovering the remaining territories" held by the Houthis, and he called on the international community to support his government.

Hours later, Saudi military spokesperson Maj Gen Turki al-Malki promised "full operational support" to Yemeni government forces until the Houthis are "eliminated".

He also said Saudi Arabia had the right to take all necessary measures to deter the rebels, who have continued firing missiles and drones at the kingdom during the nearly three-month escalation. The latest flare-up began in mid-July, when the Houthis accused Saudi Arabia of bombing the airport in Sanaa, the rebel-held capital, to stop an unauthorised Iranian flight from landing. The rebels then launched missile and drone attacks on Saudi Arabia and Yemeni government forces, announced a blockade on Saudi shipping in the Red Sea, and attacked several Saudi-linked vessels.

In September, they stepped up their campaign with their biggest territorial gains in years along the Red Sea coast. The Institute for the Study of War, a Washington-based think tank, said the capture of Turbah "will likely severely constrain Yemeni government-aligned forces' ability to reinforce and resupply front-line positions" across Taiz province and the provincial capital. It added: "Disrupting resupply has military value, of course, but controlling all the roads in and out of areas to the north of Turbah also prevents government forces from retreating out of the city."

The escalation has revived a war that began in 2014, when the Houthis swept down from their northern stronghold, captured Sanaa and forced the government into exile in Saudi Arabia. The conflict has caused one of the world's worst humanitarian disasters and killed more than 150,000 people. According to the United Nations, the latest violence has killed more than 800 people and injured over 3,600, while more than 145,000 have been displaced within Yemen and over 3,000 have fled across the water to Djibouti.