Yemen’s warring parties could start withdrawing forces from the main port city of Hodeidah within weeks, a move needed to pave the way for political negotiations to end the four-year war, the U.N. special envoy said on Thursday.

The United Nations has struggled to implement a pact agreed at talks last December in Sweden, the first major breakthrough in peace efforts to end the war that has killed tens of thousands and pushed Yemen to the brink of famine.

“The two parties agreed formally to the concept of operations for phase one. What we are doing now is … moving on as planned from there to agree on phase two,” Griffiths told Reuters in a telephone interview without elaborating, adding that talks would “intensify” in coming days.

“So we don’t have an exact date at the moment for the beginning of this physical redeployment,” he said. “It’s got to be weeks … hopefully few weeks.”

Sources have told Reuters the first phase would see the Houthis leave the city’s ports and pro-government forces leave some areas on the city’s outskirts. In the second phase, both sides would pull troops to 18 km from the city and heavy weapons 30 km away.

The Hodeidah deal was a trust building step aimed at averting a full-scale assault on Hodeidah by the Saudi-led coalition trying to restore the internationally recognised government of Abd-Rabbu Mansour Hadi, and paving the way for political talks to set up a transitional government.

Danish general Michael Lollesgaard, head of the U.N. observer team in Hodeidah, chairs a Redeployment Coordination Committee (RCC) tasked with hammering out details not spelled out in the pact.

A ceasefire in Houthi-held Hodeidah has largely held but violence has escalated elsewhere in the country. The troop withdrawal was due to have been completed by Jan. 7 but stalled over disagreement on who would control the Red Sea port city.

Asked if that issue had been resolved, Griffiths said: “We have ideas on how to bridge the gap on the issue of the local security forces” but it would be up to the parties represented in the RCC headed by Lollesgaard to resolve it.

Three sources told Reuters last month that the first phase would see the Houthis pull back 5 km (3 miles) from the ports of Saleef, used for grain, and Ras Isa, for oil. Then the Houthis would quit Hodeidah port while coalition forces would retreat 1 km from the city’s “Kilo 8” and Saleh districts.

This would restore access cut off since September to the Red Sea Mills, which holds some 50,000 tonnes of World Food Programme grain, enough to feed 3.7 million people, and allow humanitarian corridors to be reopened.

FAILURE “NOT AN OPTION”

Hodeidah handles the bulk of Yemen’s commercial and aid supplies and is critical for feeding the population of 30 million people. It became a focus of fighting last year, raising concern that an all-out assault could disrupt supply lines and trigger mass starvation in the poorest Arabian Peninsula nation.

“I know we’re spending an enormous amount of time, and rightly so, on Hodeidah, but it’s the gateway to the comprehensive settlement and of course failure in Hodeida is not an option,” Griffiths said.

“The aim ultimately of an agreement which will resolve the conflict and end this war is to return governing of Yemen to politicians, to return to the people of Yemen accountable government.”

After years of military stalemate, the Sunni Muslim coalition led by Saudi Arabia and the United Arab Emirates twice launched offensives last year to seize Hodeidah port, seeking to weaken the Houthis by cutting off their main supply line.

The coalition says the Houthis use the port to smuggle weapons and they must relinquish it. The Houthis say the government would try to choke them off if it gained control.

Griffiths said teams from the U.N. Verification and Inspection Mechanism (UNVIM), which would inspect ships docking in the ports, are ready to deploy. Monitors and other U.N. staff would backstop customs, revenue and port authorities, he added.

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

The alliance intervened in Yemen in 2015 to restore Hadi’s government which was ousted from power in the capital Sanaa in late 2014. The Houthis deny being puppets of Iran and say their revolution is against corruption.