Gunshots reverberate through a battle-scarred granary in the port city of Hodeida, where a mountain of grain meant for starving Yemenis remains inaccessible as a hard-won ceasefire comes under strain.

The Red Sea Mills, one of the last positions seized by Saudi and Emirati-backed forces before last month's UN-brokered truce, holds wheat that could feed nearly 4 million people for a month in a country on the brink of famine.

But the facility, a shrapnel-pocked symbol of how controlling food is a weapon in Yemen's war, has remained off-limits to aid organisations since September as skirmishes shake the fragile ceasefire agreed with Houthi rebels during talks in Sweden.

The site, on Hodeida's eastern edges, was rigged heavily with mines when it slipped from Houthi control in November.

Last week, during a military embed organised by the Saudi-led coalition, AFP saw government loyalists including Sudanese mercenaries scouring the vast complex with metal detectors amid fears rebels were sneaking in to plant new booby traps.

A column of smoke snaked into the sky from Houthi positions less than 1.6 kilometres away, with loyalists saying the rebels were burning tyres in a provocative move.

Then, a volley of close-range gunshots crackled through the complex. It was not possible to tell who was firing.

"We are committed to the truce... but the enemy has not committed to anything as you can see and hear," said Yemeni commander Mohammed Salman, standing by a pile of grain.

Just after the tour, the United Nations on Friday reported apparent mortar shelling at the mill had started a fire that left two food silos damaged.

"The loss of this wheat comes at a terrible time," said the UN's humanitarian coordinator Lise Grande.

"More than 20m Yemenis, nearly 70 per cent of the entire population, are hungry."

'Red line'

In Hodeida, the entry point for more than two-thirds of Yemen's food imports and international aid, the choices are stark ─ either an imperfect truce with violations on both sides or all-out fighting that could unleash famine.

The ceasefire came after Western nations pressed Saudi Arabia to end its four-year military campaign in Yemen after journalist Jamal Khashoggi's murder intensified international scrutiny. But with recurring breaches, observers warn that its collapse could be just a matter of time.

"If it is allowed to break down, there will be no opportunity for a similar deal for a long time," the International Crisis Group said.

Inside Hodeida, a city rippling with tension as the war-wounded continue to trickle into hospitals, a common refrain among civilians and military officials is 'mafi hudna' ─ Arabic for 'no truce'.

Colonel Saeed Salmeen, an Emirati commander on Yemen's west coast, told AFP his men were committed to the ceasefire but were "always ready" for battle.

He warned that Yemen's west coast road ─ a key supply route linking the south to Hodeida ─ was a "red line".

UN chief Antonio Guterres said on January 7 that neither side had tried to gain new territory since the ceasefire.

But an agreed redeployment of rival forces from the city, a key confidence-building measure, has not happened as distrust runs high.

Guterres pointed to allegations from the Houthis that pro-government forces are massing troops near the city, and from the Saudi-led coalition that the rebels are fortifying positions with barricades and trenches.

'Food is a weapon'

"How long can the international community accept this Houthi game ─ ceasefire, regroup, ceasefire, regroup?" a member of the coalition told AFP, requesting anonymity. "Only when you catch them by the neck will they come to the negotiating table."

The sentiment was echoed by other pro-government troops who insisted to AFP that military action was the only solution even if it results in bloodshed.

But the truce has given the World Food Programme (WFP) "some breathing room" to reach districts in southern Hodeida that were previously inaccessible due to fighting, its country director Stephen Anderson told AFP.

However, 51,000 metric tonnes of wheat ─ one quarter of WFP's flour-milling capacity in Yemen ─ remains locked away in the Red Sea Mills.

"We have been trying to get access... (But I hear) the Houthis aren't allowing us to get to the mill," WFP chief David Beasley said in an interview in Davos. "So it is four steps forward, two steps back, but I am still cautiously optimistic."

The lack of access to the mill, one of the most hotly contested sites in Hodeida, is a collective punishment for starving Yemenis on both sides of the conflict.

Salman, the Yemeni commander, alleged the Houthis hoarded grain, creating artificial shortages and exacerbating famine-like conditions.

When the Houthis controlled the mill, they accused the coalition of destroying food with indiscriminate air strikes.

"The Red Sea Mills is a leverage point being used in the most Machiavellian ways by all warring parties to achieve political goals," said Wesam Qaid, executive director of Yemeni development organisation SMEPS.

"Whoever controls such facilities will have greater say on who gets fed. Food is a weapon."