* Warring parties still haven't started main port troop withdrawal
* Yemen special envoy arrives in Sanaa to salvage Hodeidah deal
* U.N. seeks $4 billion in aid at Geneva pledging conference
* Hodeidah is a lifeline for millions of Yemenis facing starvation
DUBAI/GENEVA, Feb 26 (Reuters) - A peace deal in Yemen's main port city appears to have stalled again despite U.N. efforts to salvage the pact intended to clear the way for wider negotiations to end the devastating four-year war, sources involved in the discussions said.
United Nations chief Antonio Guterres admitted at a pledging conference in Geneva on Tuesday, which seeks to raise $4 billion for Yemen, that progress has been slow in implementing a troop withdrawal in Hodeidah, a lifeline for millions facing starvation.
The Iran-aligned Houthi movement controls the Red Sea city, now a focus of the war, while other Yemeni factions backed by a Saudi-led coalition loyal to the ousted government are massed on the edges. Both sides were meant to redeploy forces by Jan. 7.
A timeline announced last week was missed. It was supposed to launch a phased approach whereby the Houthis would withdraw from two smaller ports within days, to be followed by a coalition retreat from the city's eastern suburbs.
"It is not very clear why they canceled the withdrawal as the Houthi leader himself said they are ready to redeploy unilaterally," one of the sources told Reuters.
Other sources said deep mistrust among the parties remained the main obstacle to forming a local authority that would run the city and ports according to the truce agreement reached at U.N.-led peace talks in December.
Houthi officials did not respond to a Reuters' request for comment. An official in the Saudi-backed government of Abd-Rabbu Mansour Hadi told Reuters the Houthis do not want peace.
The office of U.N. special envoy Martin Griffiths, who arrived in the Houthi-held capital Sanaa on Tuesday to salvage the deal, declined to comment.
Michael Aron, Britain's ambassador to Yemen, told Reuters in Geneva he hoped the withdrawal would take place this week.
"It really has to happen ...If there isn't implementation of Stockholm, we're not back to square one, we're back to square minus one," he said.
The deal aimed to reopen humanitarian corridors and avert a full-scale assault by the coalition to seize Hodeidah port, the entry point for the bulk of Yemen's commercial and aid imports.
Such an offensive could disrupt supply lines, risking a mass famine in the poorest Arabian Peninsula nation, which is grappling with the world's most urgent humanitarian crisis.
A truce that came into force on Dec. 18 has largely held despite skirmishes on the city's outskirts.
Guterres announced that a U.N. team on Tuesday visited a grains facility caught on a frontline where the World Food Programme has enough wheat to feed 3.7 million Yemenis for a month.
"For the first time in six months, finally it was possible for us to reach the so-called Red Sea mills..." he said. "So at least slowly some progress is being made."
The war, which has killed tens of thousands of people, and ensuing economic collapse have left 16 million facing severe hunger.
Saudi Arabia announced a $500 million contribution at the Geneva pledging conference while the U.S. delegation promised $24 million.
Saud Arabia is leading the Western-backed Sunni Muslim coalition that intervened in Yemen in 2015 against the Houthis to try to restore Hadi's government, which was ousted from power in Sanaa in 2014.
Western nations have pressed for an end to the war following increased scrutiny after the murder of prominent Saudi journalist Jamal Khashoggi at the kingdom's Istanbul consulate.
The conflict is widely seen in the region as a proxy war between Saudi Arabia and Iran. The Houthis deny receiving help from Tehran and say their revolution is against corruption. (Additional reporting by Mohammed Ghobari in Aden and Tom Miles in Geneva; Editing by Ghaida Ghantous and Ed Osmond)