DUBAI/new york - Saudi-led air strikes targeting Iranian-allied Houthi forces intensified in Yemen on Tuesday, residents said, an escalation of a nine-month-old war that follows a rise in tensions between the kingdom and arch foe Tehran.

After weeks of a relative lull, large air strikes targeted military positions linked to Yemen's ascendant Houthis in the capital Sanaa, the port city of Hodaida and the disputed southwestern city of Taiz. Heavy shelling resumed on battle fronts which had been largely static during a truce which began on Dec. 15 in tandem with United Nations-backed peace talks.

Houthi fighters launched Katyusha rockets at the city of Marib, residents said, their first attack on the area since Gulf Arab troops and armed loyalists of President Abd-Rabbu Mansour Hadi seized it from the group over the summer.

Houthi forces also advanced against government militiamen in the northwestern province of Hajja and the far southern province of Lahj, reversing recent gains made the coalition. A military coalition led by Saudi Arabia and its allies has been fighting Yemen's Houthi movement in Yemen to repel what it sees as creeping influence by the group's ally, Iran.

The kingdom on Saturday announced the end of the truce, which had reduced fighting but had been repeatedly violated by both sides. Civilians are suffering a “terrible toll” in the fighting tearing Yemen apart, with casualties now topping 8,100, nearly 2,800 of them killed, amid Saudi-led coalition airstrikes, shelling by Houthi groups and other clashes, the United Nations reported Tuesday. “Airstrikes have continued into the New Year, with around 11 strikes taking place in the capital Sana’a on Sunday and Monday (3 and 4 January), and further airstrikes are reported to have been carried out in the early hours of this morning,” UN High Commissioner for Human Rights (OHCHR) spokesperson Rupert Colville told the regular news briefing in Geneva.

He put civilian casualties recorded between 26 March and 31 December, 2015 at 8,119 people, 2,795 of them killed and 5,324 wounded, noting that at least 62 civilians were reported killed by airstrikes attributed to coalition forces in December, more than twice the number of November.

“We have also received alarming information on the alleged use of cluster bombs by coalition forces in Hajjah Governorate,” he added, reporting that an OHCHR team found remnants of 29 cluster submunitions near banana plantations in Al-Odair village in Haradh District.

“According to witnesses, several other villages in the same area have also been affected. Our team also documented the use of cluster submunitions in several other districts, including Hairan and Bakel Al-Meer, and interviewed two patients who had reportedly been wounded, in separate incidents, after stepping on unexploded submunitions.” During December, at least 11 civilians were allegedly killed by shelling attributed to members of the Popular Committees affiliated with the Houthis, a significant decrease compared to 32 reportedly killed by them in November, Colville said.

The UN has been trying to broker an end to the fighting but these efforts have been stymied by violations of the ceasefire required to get the process under way. In December UN Special Envoy Ismail Ould Cheikh Ahmed adjourned peace talks until mid-January to allow for bi-lateral in-country and regional consultations to achieve a ceasefire.

The spokesman voiced particular concern at the situation in the central city of Taiz, scene of virtually uninterrupted violent clashes for more than eight months, where strict control of all entry points by Houthi-affiliated Popular Committees has limited access to essential items, including food, and made conditions extremely difficult for the civilian population.

The health situation in Taiz governorate has also continued to deteriorate, with Al-Rawdha Hospital, one of the largest still operating, forced to turn patients away. The prison system has been heavily impacted, he added, with over 40 prisoners reportedly killed and 10 others injured by airstrikes or indiscriminate shelling. More than 4,300 prisoners have reportedly escaped from facilities across the country, including those in Sa’ada, Al Dhale’e and Aden, after they were hit by airstrikes or breached in armed clashes.

Prisoners are increasingly vulnerable. Food, electricity, water and fuel shortages have been reported in many detention facilities as well as the spread of contagious diseases, such as scabies. Many detention facilities are also severely overcrowded.

"During the month of December, at least 62 civilians were reported to have been killed by airstrikes attributed to the coalition forces," Rupert Colville, spokesman for the UN human rights agency, told reporters.

That number, he pointed out, was more than double the 29 civilians reported killed in such strikes a month earlier.

The number of civilians killed by the Iran-backed Huthi rebels and their allies was meanwhile cut by two thirds to at least 11 in December from 32 a month earlier, Colville said. The remaining eight civilian deaths last month have not yet been conclusively attributed to either side, he told AFP.

The dramatic increase in the number of civilians killed in airstrikes came despite a ceasefire declared on December 15, at the same time as UN-backed peace talks between the warring sides began in Switzerland.

But the talks ended five days later with no major breakthrough, and the ceasefire collapsed on January 2 after being violated on a daily basis.

Colville pointed out that UN rights agency staff had begun receiving reports of violations "within minutes of the ceasefire beginning."

The ceasefire certainly did little to shield civilians. On December 18, he pointed out, 18 civilians were allegedly killed when two airstrikes hit a civilian house in Wadi Kena, in Saada, and two days later, six civilians, including three children, were killed in strikes on a residential neighbourhood in Al Hudayhda City.

The airstrikes have continued into the new year, with some 11 strikes taking place in Sanaa on Sunday and Monday alone, with reports of further airstrikes Tuesday morning, Colville said, pointing to reports indicating that civilian buildings had been hit in densely populated areas of Sanaa.

Colville also decried "alarming information" that coalition forces used cluster bombs in Hajjah governorate, saying that a field visit by UN rights office staff last month had found remnants of 29 cluster submunitions near banana plantations in the village of al-Odair in Haradh district.

He said other villages and other districts also appeared to have been affected.

Colville also voiced particular concern at the humanitarian situation in violence-wracked Taez, lamenting that "strict control of all entry points into the city by the popular committees affiliated with the Huthis has resulted in limited access to essential items, including food."

Yemen's conflict erupted in September 2014, when the Huthis advanced from their northern strongholds to occupy the capital Sanaa.

Since the conflict escalated dramatically when the Saudi-led air strikes began in March, at least 2,795 civilians have been killed and 5,324 wounded, Colville said.

A new round of peace talks have been scheduled for January 14 in an unspecified location.

And UN special envoy Ismail Ould Cheikh Ahmed is set to visit Riyadh on Wednesday amid fears that a diplomatic storm unleased by Saudi Arabia's break in relations with Iran could thwart the efforts to end Yemen's conflict.