SANAA, Dec 16 (Reuters) - A ceasefire agreed between Yemen's warring parties in Hodeidah will begin on Dec. 18, sources from both sides and the United Nations said on Sunday, to try to avert more bloodshed in a port city vital for food and aid supplies.The Iranian-aligned Houthi group and the Saudi-backed government of Abd-Rabbu Mansour Hadi agreed on Thursday after a week of UN-sponsored peace talks in Sweden to cease fighting in the Red Sea city and withdraw forces.The Houthis control most towns and cities, including Hodeidah and the capital Sanaa, from where they ousted Hadi's government in 2014 prompting a Saudi-led coalition to intervene. The government is now based in the southern port of Aden.Residents have reported continued skirmishes, mostly at night, on the outskirts of Hodeidah, where thousands of coalition-backed Yemeni troops have massed.Yahya Sarea, a senior official of the Houthi armed forces, told reporters in Sanaa the ceasefire was set to start on Tuesday."We hope they will be true to their words, otherwise we are ready to respond," he added.A source in the Saudi-backed government confirmed the date and said it was officially communicated to both parties in a letter from special Yemen envoy Martin Griffiths."While the Hodeidah agreement states an immediate start of the ceasefire, it is normal that it takes 48-72 hours to be communicated at the operational level," a UN source said. "We expect the ceasefire to be implemented starting Tuesday."