1.2236081-857304108
An armed personnel carrier belonging to the Yemeni government troops head to the frontline in Hodeida. Image Credit: Supplied

Yemen: A Saudi-led coalition backing Yemen's exiled government began an assault on Wednesday morning on Yemen's port city of Hodeida, a crucial battle in the three-year-old conflict that aid agencies warned could push the Arab world's poorest country into further chaos.

Iranian-aligned Al Houthis and their allies for years have held the Red Sea port, crucial to food supplies in a nation on the brink of famine after years of war. The battle for Hodeida, if the Houthis don't withdraw, also may mark the first major street-to-street urban fighting for the Saudi-led coalition, which can be deadly for both combatants and civilians alike.

Before dawn on Wednesday, convoys of vehicles appeared to be heading toward the rebel-held city, 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 and later state media announced the battle had begun, citing military sources. They also reported coalition airstrikes and shelling by naval ships.

Yemen's exiled government "has exhausted all peaceful and political means to remove the Houthi militia from the port of Hodeida", it said in a statement. "Liberation of the port of Hodeida is a milestone in our struggle to regain Yemen from the militias."

The Houthi-run Al Masirah satellite news channel later acknowledged the offensive, claiming rebel forces hit a Saudi coalition ship near Hodeida with two missiles. Houthi forces have fired missiles at ships previously.

"The targeted ship was carrying troops prepared for a landing on the coast of Hodeida," the channel said.

The Saudi-led coalition did not immediately acknowledge the incident. The US Navy's 5th Fleet, whose area of responsibility includes the Red Sea, referred questions to the Pentagon, which did not immediately respond to a request for comment.

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 kilometres southwest of Sanaa, Yemen's capital held by the Houthis since they swept into the city in September 2014. The Saudi-led coalition entered the war in March 2015 and has received logistical support from the US.

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.

More than 10,000 people have been killed in Yemen's civil war, which has displaced 2 million more and helped spawn a cholera epidemic. 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.

Before the war, more than 70 per cent of Yemen's food and fuel imports came through Hodeida, accounting for over 40 per cent 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. A Saudi-led airstrike in 2015 destroyed cranes at Hodeida. The UN in January shipped in mobile cranes to help unload ships there.

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.

UN Secretary-General Antonio Guterres had said that UN envoy Martin Griffiths was in "intense negotiations" in an attempt to avoid a military confrontation. However, Griffiths' recent appointment as envoy and his push for new negotiations may have encouraged the Saudi-led coalition to strengthen its hand ahead of any peace talks with the Houthis.

The attack also comes as Washington has been focused on President Donald Trump's recent summit with North Korean leader Kim Jong-un. A statement from Secretary of State Mike Pompeo said Monday he spoke with Emirati officials and "made clear our desire to address their security concerns while preserving the free flow of humanitarian aid and life-saving commercial imports".

Defence Secretary Jim Mattis on Monday acknowledged that the US continues to provide support to the Saudi-led coalition.

"It's providing any intel, or anything we can give to show no-fire areas where there are civilians, where there's mosques, hospitals, that sort of thing - (and) aerial refueling, so nobody feels like I've got to drop the bomb and get back now," he said.

It wasn't immediately clear what specific American support the coalition was receiving Wednesday.