DUBAI/LONDON (Reuters) - The United Nations envoy for Yemen on Monday urged the warring parties to withdraw their troops from the port of Hodeidah quickly, and international aid agencies said conditions for thousands of starving people were deteriorating fast.

Envoy Martin Griffiths acknowledged that proposed timelines on a pullout from the port, the main entry point for Yemen’s commercial and aid imports, had slipped while the country stood on the brink of famine.

“The initial timelines were rather ambitious,” he said in comments posted on Twitter. “We are dealing with a complex situation on the ground.”

The aid agencies, meeting in London, said people were struggling to feed their children in what had become the world’s worst humanitarian crisis.

But, said Isabelle Moussard Carlsen of Action Against Hunger, more aid was not the only solution to alleviate the suffering of the Yemeni people. “I think we need to be very clear that we need a political solution to this conflict,” she said.

Agreements reached in December between the Iranian-aligned Houthi movement and the Saudi-backed government of Abd-Rabbu Mansour Hadi were the first significant breakthrough in four years of conflict which have killed tens of thousands of people through military actions or other causes.

But little further progress has been made, risking the unraveling of the peace efforts.

The Houthis control Hodeidah, and troops of a Saudi-led coalition are massed on its outskirts. The warring sides disagree over who should control the city and port after forces withdraw.

The truce in Hodeidah has largely been respected since coming into force a month ago, but skirmishes continue. Troops have not yet pulled out, missing a Jan. 7 target, and residents and aid workers have told Reuters that barricades, trenches and roadblocks have been reinforced.

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U.N. Secretary-General Antonio Guterres notified the Security Council on Monday, in a letter seen by Reuters, that he would appoint a new head of the U.N. monitoring mission tasked with overseeing the Hodeidah peace deal.

Retired Dutch general Patrick Cammaert, who arrived in Hodeidah late last month to lead an advance team of monitors, will be replaced with Danish Lieutenant General Michael Anker Lollesgaard, who led a U.N. peacekeeping mission in Mali in 2015 and 2016, Guterres said. He did not state why Cammaert was leaving.

A senior U.N. diplomat, speaking on condition of anonymity, said last week that Cammaert had said he would do the job only for a short time.

PRISON WITHOUT WALLS

Although fighting has escalated in other parts of Yemen, U.N. envoy Griffiths said he remained optimistic.

“More than any time in the past, there is a political will demonstrated by all parties to put an end to this conflict,” he said. “What we need to see now is the implementation of the provisions of the agreement, fully and rapidly.”

The war has been stalemated for years, with the Saudi-led coalition of Sunni Muslim Arab states and Yemeni allies unable to dislodge the Houthi movement that controls Sanaa, the capital, and most urban centers.

The coalition has twice attempted to capture Hodeidah port since last year to force the Houthis to negotiate, but held off from a full-blown assault amid fears that a disruption to supply lines would trigger mass starvation.

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At the London meeting, 14 aid agencies called for action on the humanitarian crisis.

“It is what I like to call a prison without walls for the people living in the country at the moment. It is a difficult situation where people are struggling to buy their daily rations to be able to feed their children,” Yemeni Oxfam campaign manager Awssan Kamal told Reuters.

Kimberley Brown of the British Red Cross said 85,000 children had lost their lives and malnutrition was taking a huge toll. “I know from my colleagues that the situation is absolutely deteriorating at the moment,” she said.

The coalition intervened in 2015 in Yemen to restore Hadi’s government, which was ousted from power by the Houthis in late 2014. The group says its revolution is against corruption.

Western nations, some of which supply the coalition with arms and intelligence, are pressing for an end to the conflict that is widely seen in the region as a proxy war between Saudi Arabia and Iran.