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.
Residents say they have heard shelling and air raids outside the city.
The Saudi-led coalition backing Yemen's exiled government began an assault Wednesday morning on Hodeida.
Yemeni officials say the battles are centered in and around the ad-Durayhimi district. They say a network of minefields in the rebel-held areas has hindered the advance of government forces.
The officials and residents spoke on condition of anonymity because they were not authorized to brief the media and for fear of reprisals.
Ahmed al-Haj in Sanaa.
Yemeni officials say Saudi-led coalition airstrikes have hit most of the Houthi rebel defenses of Hodeida as part of the coalition's assault on the key Red Sea port town.
They say the fighting is raging in the coastal areas of ad-Durayhimi district.
Residents of coastal villages and other areas between ad-Durayhimi and southern Hodeida had left their homes ahead of the fighting.
The officials said government forces have headed to the eastern coast aiming at cutting off a rebel supply line between Hodeida and Sanaa. Naval forces, they said, are ready to join the fighting from the southern side of Hodeida.
The officials spoke on condition of anonymity because they were not authorized to brief the media.
The officials say the forces assigned to break into the city are comprised of three brigades of UAE-backed Amaleqa bridges.
One official says more than 2,000 troops trained in a military base in Eretria, have arrived at the port of Hodeida, assigned to secure the facility.
Ahmed al-Haj in Sanaa.
Yemen's exiled government says their forces and allied Saudi-led troops launched their assault on Hodeida only after "exhausting all peaceful and political means."
The statement on the government-controlled SABA news agency early Wednesday morning called the battle for Hodeida "a milestone in our struggle to get Yemen back from the militias."
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. A Saudi-led coalition entered the conflict in March 2015.
The Houthis did not immediately acknowledge the start of the battle.
Saudi-owned satellite news channels have announced the start of the Saudi-led coalition's assault on Hodeida.
Citing sources within the military, Dubai-based Al Arabiya said the battle began early Wednesday morning.
Al Arabiya is believed to be at least partially owned by Saudi Arabia's government.
There was no immediate word from state media in either Saudi Arabia or the United Arab Emirates, which lead a coalition backing Yemen's exiled government.
Gunfire has erupted near Yemen's port city of Hodeida as a Saudi-led coalition's deadline expired for Shiite rebels there to withdraw.
Social media users shared video of what appeared to be a convoy of vehicles approaching the crucial port city early Wednesday morning. The sound of heavy, sustained gunfire clearly could be heard.
Forces loyal to Yemen's exiled government and irregular fighters led by Emirati troops had neared Hodeida in recent days.
Emirati Minister of State for Foreign Affairs Anwar Gargash earlier told French newspaper Le Figaro the deadline for a withdraw from Hodeida by Shiite rebels known as Houthis expired early Wednesday morning.
The United Nations and other aid groups had pulled their international staff from Hodeida ahead of the rumored assault.
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