The United Nations’ special envoy for Yemen has called on the country’s warring sides to pull their forces out of the Red Sea port city of Hudaydah as soon as possible under a recent UN-brokered ceasefire.

Martin Griffiths tweeted on Monday that the timelines set for the withdrawal of Houthi fighters and Saudi-backed militants loyal to the ex-Yemeni government from Hudaydah had slipped.

Representatives from the Houthi Ansarullah movement and the Riyadh-sponsored government of ex-president, Abd Rabbuh Mansur Hadi, reached the truce deal during UN-mediated peace talks in Sweden last month.

Under the deal, they agreed to the withdrawal of their troops and the deployment of UN monitors to the port city, a lifeline for millions of Yemenis.

However, the Houthis – who control Hudaydah -- have repeatedly complained that the Saudi-led coalition, which has been waging a deadly war on Yemen since March 2015, has been violating the ceasefire.

The two sides have also exchanged the names of some 16,000 prisoners, whose transfers were expected to begin on January 19.

“We have seen the timelines for implementation extended, both in Hudaydah, and the prisoner exchange agreement. Such changes in timelines are expected,” Griffiths said. “The initial timelines were rather ambitious. We are dealing with a complex situation on the ground.”

Separately on Monday, the UN envoy told London-based Asharq al-Awsat newspaper that he was optimistic about the resolution of the Yemen conflict.

“We have seen the two parties demonstrate remarkable political will, first to reach a ceasefire agreement, and then to abide by it. What we need to see now is the implementation of the provisions of the agreement, fully and rapidly,” he said.

The official further confirmed that retired Dutch General Patrick Cammaert, who has been leading an advance UN mission tasked with overseeing the Hudaydah truce, was stepping down. He, however, denied reports that disagreements had led to the resignation.

“There is no element of truth to such reports. As a matter of fact, General Cammaert and I have been working closely to close the gap between the two parties on the operational implementation of the Hudaydah Agreement,” Griffiths noted.

Earlier on Monday, UN Secretary General Antonio Guterres notified the Security Council, in a letter seen by Reuters, that Cammaert would be replaced with Danish Lieutenant General Michael Anker Lollesgaard, who led a UN peacekeeping mission in Mali in 2015 and 2016.

Meanwhile, Ansarullah leader Abdul-Malik Badreddin al-Houthi sat down with Griffiths in the capital Sana’a on Monday.

During the meeting, Houthi criticized the Saudi-led aggressors for maintaining the inhumane siege on Yemen, closing the Sana’a airport and taking unjust measures against Yemeni civilians.

Griffiths, for his part, vowed that he would take serious steps in the coming days regarding the implementation of the Hudaydah ceasefire and the planned prisoner swap.

Aid groups raise concerns

In another development on Monday, 14 international aid agencies met in London, warning that conditions for thousands of starving Yemeni people were deteriorating fast.

WATCH LIVE NOW: Yemen cannot wait conference starting now - 14 aid agencies calling on urgent action from ground in Sanaa and Hodeida @Oxfam @save_children @Federation @ACF_UK https://t.co/tlD8wt2Drv pic.twitter.com/fkIFSVZ7Ap — Islamic Relief (@IRWorldwide) January 28, 2019

“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,” said Yemeni Oxfam campaign manager Awssan Kamal.

Moreover, Isabelle Moussard Carlsen of Action Against Hunger underlined the need for a political solution to the conflict.

Kimberley Brown of the British Red Cross noted that an estimated 85,000 Yemeni children had lost their lives and malnutrition was taking its toll, warning, “The situation is absolutely deteriorating at the moment.”