DUBAI, United Arab Emirates (AP) — A Saudi-led coalition backing Yemen's exiled government began an assault Wednesday morning on Yemen's port city of Hodeida, a crucial battle in the 3-year-old conflict that aid agency warned could push the Arab world's poorest country into further chaos.

Before dawn, convoys of vehicles appeared to be heading toward the rebel-held city on the Red Sea, according to videos posted on social media. The sound of heavy, sustained gunfire clearly could be heard in the background.

Saudi-owned satellite news channels later announced the battle had begun, citing military sources. State media in both Saudi Arabia and the United Arab Emirates did not immediately acknowledge the assault.

Forces loyal to Yemen's exiled government and irregular fighters led by Emirati troops had neared Hodeida in recent days. The port is some 150 kilometers (90 miles) southwest of Sanaa, Yemen's capital held by Shiite rebels known as Houthis.

Emirati Minister of State for Foreign Affairs Anwar Gargash earlier told French newspaper Le Figaro the deadline for a withdrawal from Hodeida by the Houthis expired early Wednesday morning.

The United Nations and other aid groups already had pulled their international staff from Hodeida ahead of the rumored assault.

Over 10,000 people have been killed in Yemen's civil war. The Saudi-led coalition has been criticized for its airstrikes killing civilians. Meanwhile, the U.N. 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.

Before the war, over 70 percent of Yemen's food and fuel imports came through Hodeida, accounting for over 40 percent of the nation's customs income. The port remains crucial for incoming aid, food and medicine for a nation driven to the brink of famine by the conflict and a Saudi-led blockade.

The U.N. 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.