Yemen's Houthis deployed additional forces in the main port city of Hodeida yesterday as a Saudi-led military coalition moved closer to the city center in the largest offensive of the war, raising U.N. fears of a humanitarian catastrophe.

The alliance led by Saudi Arabia and the United Arab Emirates launched its assault on the heavily defended Red Sea city on June 12 to try to weaken the Iran-aligned Houthi movement by cutting off a key supply line for the group which controls the capital Sanaa and most populated areas.

"There is a heavy deployment of armed Houthis in the city and new check points have been set up in neighborhoods where there are supporters of the Tehama brigades," said one resident, referring to a Yemeni faction from the Red Sea coastal plain that is fighting with coalition forces.

Fierce clashes broke out after midnight near Hodeida University, around 3 km (1.9 miles) west of the city center, on the coastal road linking the airport to the port, added the resident, who requested anonymity.

Coalition forces seized the airport on Wednesday and have been consolidating their hold in the area as U.N. efforts continued to reach a political deal that would avert an assault on the port, a lifeline for millions of Yemenis.The United Nations fears the escalation in fighting could exacerbate what is already the world's most urgent humanitarian crisis, with 22 million Yemenis dependent on aid and an estimated 8.4 million believed to be on the verge of starvation.