Yemen's warring sides have accused each other again of violating a hard-won truce in the battleground city of Hodeida. The Saudi-led coalition warned on Wednesday that the two-day-old ceasefire could collapse if the rebels' alleged violations persist and the United Nations does not intervene.

But in spite of the finger pointing over sporadic shelling and gunfire that marred the peace Tuesday night in Hodeida, the city was reportedly calm again by Wednesday morning, and there was international help on the way to try and maintain the peace. Meanwhile, progress was being made on a major prisoner exchange which, if it goes ahead, could help to solidify the good will generated by weeks of diplomacy.

United Nations observers were due to arrive in the Red Sea port city on Wednesday to chair monitoring teams made up of Yemen government and rebel representatives tasked with overseeing the implementation of the ceasefire that took effect on Tuesday.

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"A total of 21 violations since ceasefire commencement have come to our notice," a coalition source told AFP and other news agencies on condition of anonymity. "If the U.N. continues to drag the chain and take too long to get into the (military) theatre, they will lose the opportunity altogether... and the agreement will turn a dead duck."

"We will continue to give them the benefit of the doubt and show restraint but early indicators are not promising," said the source, who spoke in English on condition of anonymity to the AFP and Reuters news agencies.

The Houthi-controlled al-Masirah TV, however, accused the Saudi coalition of violating the ceasefire on Tuesday night by firing shells at rebel territory near Hodeida's airport.

U.N. observers expected in Yemen soon

A U.N. official had told AFP on Tuesday that the observers would deploy "within 24 hours" in the city whose strategic docks are the point of entry for 80 percent of Yemeni imports and virtually all U.N.-supervised humanitarian aid.

The observers of the U.N.-chaired Redeployment Coordination Committee are due to oversee the withdrawal of the warring parties from the city, including a rebel pullout from the docks.

The committee chair is expected to report to the U.N. Security Council on a weekly basis as part of a major diplomatic push to end the four year-old conflict that is seen as the best chance yet for peace.

Hodeida residents reached by telephone said there was complete calm in the city on Wednesday morning following intermittent gunfire during the night.

The war between the the Huthi Shiite rebels and troops loyal to President Abedrabbo Mansour Hadi escalated in 2015, when he fled into Saudi exile and the Saudi-led military coalition intervened. Since then, the war has killed some 10,000 people, according to the World Health Organization, although human rights groups say the real death toll could be five times as high.

The conflict has also pushed 14 million people to the brink of famine in what the United Nations describes as the world's worst humanitarian crisis.

Prisoner swap in the works

Reuters cited the International Committee of the Red Cross on Wednesday as saying Yemeni government and Houthi rebel officials had made significant progress toward implementing a deal for the exchange of thousands of prisoners -- a step which could greatly boost trust among the warring parties and help the ceasefire stick.

Fabrizio Carboni, ICRC regional director for the Middle East, told Reuters that the two sides had exchanged lists containing the names of about 16,000 prisoners as the next step forward following an agreement reached last week.

Carboni said the ICRC would be given access to those detainees in late January for pre-handover interviews. He said the detainees include prisoners held outside of Yemen, and some foreign nationals held inside the war-torn country.