UNITED NATIONS (Reuters) - The United Nations’ Security Council should consider “stronger measures” if there was no progress in the coming weeks on a deal between Yemen’s warring parties on a key port, Britain said on Monday, as the U.N. mediator admitted it was “taking longer than we had hoped.”

United Nations envoy to Yemen Martin Griffiths, gestures as he arrives to a meeting with Houthi-appointed local officials in Hodeidah, Yemen January 29, 2019. REUTERS/Abduljabbar Zeyad

In an illustration of the delay, U.N. Yemen envoy Martin Griffiths told the council both sides had accepted a detailed operational plan for step one in a withdrawal of forces from the Red Sea port and city of Hodeidah - two months after the world body announced an initial deal on that measure.

“Let us be clear that when - and I hope it is when, not if - these redeployments happen they will be the first voluntary withdrawals of forces in this long conflict,” Griffiths told the 15-member council. “Of course it has taken longer than we had hoped but that it should happen at all is extremely welcome.”

The Iran-aligned Houthi movement and the Saudi-backed government initially agreed in December to withdraw troops by Jan. 7 from Hodeidah - a lifeline for millions facing famine - under a truce aimed at averting a full-scale assault on the port and paving the way for negotiations to end the four-year war.

Although the ceasefire agreed for Hodeidah largely holds, violence continues elsewhere and has escalated in recent weeks.

Britain’s U.N. Ambassador Karen Pierce said the lack of implementation of the deal reached in Sweden is “very worrying.”

“The stakes are too high for us to let that fail and the council in our view should be ready to take stronger measures if there’s no progress by our next meeting,” Pierce told the council, which traditionally meets once a month on Yemen.

Griffiths did not say when the first stage of troop withdrawals from Hodeidah could start, adding that talks would continue on phase two of a redeployment and the status of local security forces.

Acting U.S. Ambassador to the United Nations Jonathan Cohen warned the Houthis that Washington would be “watching closely to see if they make good on that agreement” to phase one.

“The Yemeni government has demonstrated a clear commitment to the United Nations-led process. It’s time for the Houthis to also show the international community that they too are serious,” Cohen told the council.

U.N. aid chief Mark Lowcock painted a dire picture of the humanitarian situation in Yemen, appealing for more money for “the world’s largest aid operation” and more access.