DUBAI/ADEN, Jan 25 - Yemen's warring parties have failed to pull troops from the main port under a month-old truce, putting the first major diplomatic breakthrough of the four-year war in jeopardy and reviving the threat of an all-out assault that could unleash famine.
The resignation this week of the U.N. official monitoring the ceasefire, who quit days after his convoy was shot at, has hammered home the potential for the peace deal to collapse. If fighting restarts in earnest around the port of Hodeidah, the main supply route into the country could be cut off, leaving no way to feed millions of people on the verge of starvation.
"These coming weeks are make or break for the conflict. We will either see a restart of the political track, or we will likely see a significant military escalation," said Adam Baron of the European Council for Foreign Relations.
The truce itself has largely held in the port of Hodeidah since coming into force a month ago, but late on Wednesday clashes at flashpoints on the city's edges intensified.
And the withdrawal of troops that was meant to take place by Jan. 7 has stalled. Pulling out troops was seen as a pivotal confidence-building measure that would build up the trust needed for political talks.
Without it, U.N. Secretary-General Antonio Guterres acknowledged last week, "lack of trust" had become a "complicating factor" in trying to get the parties to talk.
The war has largely been stalemated for years, with a Saudi-led coalition of Arab states and Yemeni allies unable to dislodge the Iran-allied Houthi movement that controls the capital and most major population centers.
Last year the Arab states attempted to capture Hodeidah, hoping to seize control of the country's main supply route and finally force the Houthis to their knees.
But they failed in two attempts to capture the port, holding off from a full-blown assault that could have caused mass starvation. With the Houthis still in control the city and the Arab forces dug-in on the outskirts, they finally agreed a ceasefire at talks at a castle near Stockholm last month.
The agreement also foresees a political track of talks to end the war. But a lack of progress could test the patience of the United Arab Emirates, which leads military operations on Yemen's Red Sea coast for the Saudi-led coalition.
"People are worried that the war will start again after failure in implementing the deal," said government employee Abdullah Abdul-Bari, a 51-year-old resident of Hodeidah.
This week's resignation of the head of the U.N. mission tasked with overseeing the deal, Patrick Cammaert, came after mediators failed to convene a meeting to discuss the redeployment of forces from Hodeidah.
The Redeployment Coordination Committee had met twice in Houthi-run territory, but attempts to convene a third meeting in areas held by coalition forces failed because the Houthis were unwilling to cross the frontline, sources told Reuters.
The Houthis accused Cammaert of bias against them. Houthi spokesman Mohammed Abdul-Salam said that unless Cammaert's boss, U.N. envoy Martin Griffiths, dealt with it, "it would be difficult to talk about anything else."
Griffiths has been shuttling in recent days between the Houthi-held capital Sanaa and Riyadh to try to rescue the deal and build on progress made on a prisoner swap agreed in Stockholm but not yet finalized.
"It is a very fragile moment. People are still politically and rhetorically committed to the Stockholm Agreement as the best way forward. The question is whether we can get tangible progress on the ground," said International Crisis Group analyst Elizabeth Dickinson.
The Arab states show increasing signs of running out of patience. UAE Minister of State for Foreign Affairs Anwar Gargash on Wednesday blamed the Houthis' "obdurate behavior."
A diplomat in the Gulf said trust had been further eroded after a Jan. 10 Houthi drone attack on a Yemeni government military parade, which was followed by coalition air strikes on Houthi military targets in Sanaa.
A source familiar with the matter said the UAE felt the need to renew pressure on the Houthis.
"It is clear the UAE wants to find an exit strategy," a senior diplomat said. "This war is really costing a lot, not only in terms of economic resources but also credibility and human lives."
Western nations, many of which supply arms and intelligence to the coalition, have pressed for an end to the conflict, especially since the murder of Saudi journalist Jamal Khashoggi in October intensified scrutiny of Saudi regional policies.
Residents and aid workers said barricades, trenches and roadblocks continue to be built and reinforced in Hodeidah. The tense frontlines have made it difficult to set up humanitarian corridors to assist 10 million Yemenis facing starvation.
"Some food and fuel is coming through Hodeidah Port and moving across the country but main roads are closed, diverting it over longer routes, and fighting is ongoing, threatening the safety of transportation," the Norwegian Refugee Council's Suze van Meegan said.
The World Food Programme is still unable to reach 51,000 tonnes of wheat - enough to feed 3.7 million people for a month - stuck since September in a no-man's land in Hodeidah.
Frank McManus of the International Rescue Committee said access had become more restricted in Hodeidah in the past two weeks. In one case, medicines could not be delivered to certain health clinics. In another, a mobile health team was not given permission to visit some locations."Right after the Stockholm agreement we saw an improvement in the security situation but those days are gone," he said.