The battle for Hudaydah, which is held by the Houthi movement, is expected to be by far the biggest of a three-year war between an alliance of Arab states and the Iran-backed Houthis. The sound of heavy, sustained gunfire could be heard clearly in the background. The port remains crucial for incoming aid, food and medicine. Houthi media did not immediately report the attack.
"The liberation of Hudaydah port is a turning point in our struggle to recapture Yemen from the militias that hijacked it to serve foreign agendas", the exiled government said in a separate statement carried by state-run Yemeni media.
Forces loyal to Yemen's exiled government and irregular fighters led by Emirati troops have neared Hodeida in recent days. The port is some 150 kilometres southwest of Sanaa, Yemen's capital held by Shiite rebels known as Houthis. The Saudi-led coalition entered the war in March 2015 and has received logistical support from the US.
"We gave United Nations special envoy Martin Griffiths 48 hours to convince the Houthis to withdraw from the port and city of Hodeidah", UAE Minister of State for Foreign Affairs Dr Anwar Gargash told France's Le Figaro newspaper.
The civil war in Yemen has killed about 10,000 people over the past three years and created what the United Nations says is the world's worst humanitarian disaster. Meanwhile, the UN and western nations say Iran has supplied the Houthis with weapons from assault rifles up to the ballistic missiles they have fired deep into Saudi Arabia, including at the capital, Riyadh. The Saudi-led coalition has been criticised for its air strikes killing civilians.
The UN says some 600,000 people live in and around Hodeida, and "as many as 250,000 people may lose everything - even their lives" in the assault.