A ceasefire in Yemen's battleground port city of Hodeida and its surroundings will start tomorrow, a UN official said, after renewed fighting threatened the hard-won accord struck in Sweden.

The deal announced last Thursday between Yemen's Saudi-backed government and the Huthi rebels included an "immediate ceasefire" in Hodeida, whose Red Sea port serves as a crucial gateway for humanitarian aid.

The UN official, who requested anonymity, told AFP that the delay to the halt in hostilities until midnight (9pm GMT) tonight was necessary for "operational reasons".

Yemen's Foreign Minister Khaled al-Yamani, who led the government's delegation to the peace talks, also told state-run television late Sunday that the ceasefire would begin at midnight Monday.

Residents in Hodeida and the surrounding areas have reported fierce fighting and air strikes in recent days, as clashes continued between Saudi-backed government forces and the Iran-aligned Huthis.

At least 29 fighters, including 22 Huthi rebels, were killed on Saturday night in Hodeida province, a pro-government military source said.

Two Hodeida residents reached by telephone told AFP that they could hear intermittent clashes to the east and south of the city on Monday.

A pro-government military official said that there were sporadic clashes, adding that a fire erupted in one of the factories in the east of the city due to strikes on Sunday night.

Doctors Without Borders (MSF) voiced alarm about "the continuous fighting" in Hodeida.

The medical aid group said its teams on the ground were treating victims of gunshots, shelling and air strikes, urging warring parties "to respect the presence of civilians and health infrastructures".

UN envoy Martin Griffiths said on Sunday that the UN was working with both sides to ensure the ceasefire accord was "implemented timely and properly".

The truce is supposed to be followed by the withdrawal of fighters from Hodeida within days on both sides.

A prisoner swap involving some 15,000 detainees is planned and a "mutual understanding" was reached to facilitate aid deliveries to Yemen's third city Taiz -- under the control of loyalists but besieged by rebels.

The two sides also agreed to meet again in late January for more talks to define the framework for negotiations on a comprehensive peace settlement.

Impoverished Yemen has been mired in fighting between the Huthi rebels and troops loyal to President Abedrabbo Mansour Hadi since 2014.

The war escalated in 2015 when a Saudi-led military coalition stepped in on the government's side.

The conflict has since killed nearly 10,000 people, according to the World Health Organization. But some rights groups believe the toll to be far higher.

UN chief Antonio Guterres warned Sunday that "much worse" lay in store for the impoverished country in 2019 unless its warring parties strike a peace deal and head off a humanitarian crisis.

Severe food shortages mean that a high number of Yemenis have been dying in "very dramatic circumstances", Guterres told a news conference in Doha.

Diplomats said Mr Guterres may propose a surveillance mechanism comprising 30 to 40 observers.

AFP.