Yemen’s warring parties exchanged heavy weapons fire overnight in Hodeidah, residents and military sources said, as the United Nations scrambled to salvage a ceasefire deal in the Yemeni port city that is a lifeline for millions at risk of starvation.

Axar.az reports citing foreign media that, the clashes were the heaviest since the ceasefire went into effect on Dec. 18, residents said, and came as the United Nations announced a deal setting out details of a mutual military withdrawal envisaged by the Stockholm truce accord.

Iran-aligned Houthi forces and troops backed by a Saudi-led coalition traded artillery, mortar and rocket salvoes late on Sunday and early on Monday, with explosions heard across the Red Sea city, residents said.

“The Houthis tried a surprise assault on our troops but we stopped them,” a military source from the internationally-recognized government said.

The Houthis’ Al Masirah TV accused government forces of shelling their positions without provocation.

The fighting affected Hodeidah’s usual flashpoints - the “July 7” district, four km (2.5 miles) away from the port, and southern outskirts where thousands of United Arab Emirates-backed troops are massed.

The Houthis and the government of Abd-Rabbu Mansour Hadi agreed at U.N.-sponsored talks in December to a truce and troop withdrawal from Hodeidah, the entry point for the bulk of Yemen’s humanitarian aid and commercial imports.

The ceasefire has broadly held although sporadic clashes continued as the United Nations struggled to implement a troop withdrawal, a confidence-building measure meant to clear the way for a broader peace settlement after four years of war.