Yemeni residents in the Red Sea port city of Hodeida say the Saudi-led coalition has dropped leaflets advising them to stay away from military and security points, and to stay in their homes, amid the coalition's assault.
A senior United Arab Emirates official said on Tuesday that the Iran-aligned Houthi movement must evacuate Yemen's main port of Hodeidah as troops backed by a Saudi-led coalition geared for an assault on the city.
"As has been the case since the beginning of the war, the cost of the battle for Hodeida will largely accrue to the already impoverished civilian population", the International Crisis Group warned in a report Monday.
The assault began at around 1:15 pm (1015 GMT) after Yemeni pro-government forces received a "green light" from the Saudi-led coalition, they said.
"Yemeni resistance forces have increased their readiness for combat in preparation for the battle to liberate Hodeida from the grip of the Huthi militias", Emirati state news agency WAM said.
Late Wednesday, the Saudi and Emirati governments announced what they called a "multi-faceted plan" to protect civilians in Hodeida, including establishing routes for food, medical supplies and oil shipments from Saudi Arabia's southern city of Jizan and the UAE's capital, Abu Dhabi.
Global aid agencies and the United Nations have warned the assault could shut down the vital aid route for some 70 percent of Yemen's food, as well as the bulk of humanitarian aid and fuel supplies.
The assault, part of an operation dubbed Golden Victory, began with coalition air strikes and shelling by naval ships, according to Saudi-owned satellite news channels and state media.
The two sides were also at odds over the recent deployment of UAE forces to the Yemen's Arabian Sea island of Socotra, a dispute mediated by Saudi officials.
Four Emirati soldiers were killed in Wednesday's assault, the United Arab Emirates' state-run news agency said, but gave no details of how they died.
Exiled President Abed Rabbo Mansour Hadi and many of his advisers now live in Saudi Arabia after Shiite rebels known as Houthis seized the capital, Sanaa, in September 2014. The Saudi-led coalition entered the war in March 2015.
U.N. Secretary-General Antonio Guterres has said that U.N. envoy Martin Griffiths is in "intense negotiations" in an attempt to avoid a military confrontation.
The United Nations and other aid groups had pulled their global staff from Hodeida ahead of the rumored assault. "We already gave the United Nations the chance to operate from this seaport, and (the Houthis) refused". The sound of heavy, sustained gunfire clearly could be heard in the background.
The commanders spoke to an AFP correspondent in the town of Al Jah, approximately 30 kilometres (20 miles) southeast of rebel-held Hodeida.
"We do not provide any additional support to the Saudi coalition's military operations", he said.