Fresh battles have erupted between Yemeni forces and militants loyal to Saudi Arabia in Yemen’s strategic northern city of Marib. The clashes are reported to have left 16 pro-Saudi militants killed.

Yemeni forces are seeking to liberate the city, the last stronghold for Saudi Arabia in the country, and the city’s surrounding oil fields. Sources close to the Saudi-backed militants told AFP that Yemeni forces “launched attacks on various fronts”.

According to Yemen’s al-Masirah news channel, Saudi warplanes launched some 17 airstrikes on different sites in the wider Marib province. Saudi airstrikes have been heavily condemned for killing civilians.

The United Nations Human Rights Office says “it is seriously concerned at the continued impact of fighting on civilians and targeting of civilian objects in Marib province. We urge all parties to the conflict to go back to the negotiating table and agree on a nationwide ceasefire”.

The renewed clashes in Marib comes after a diplomatic push by the United Nations and regional countries to secure a ceasefire in the country hit a brick wall. Meanwhile, Saudi Arabia confirmed that the Kingdom has been targeted by at least 17 armed drones launched by Yemeni forces.

A spokesman for Yemen’s popular Ansarullah forces had said that the movement targeted a military base in Saudi Arabia’s south western city of Khamis Mushait with an armed drone. Saudi Arabian says Khamis Mushait was targeted again with another two drones, while eight drones had been fired towards the south of the Kingdom, one of which was aimed at the city of Najran.

Over the past two years, Yemeni forces have stepped up retaliatory attacks using indigenously made missiles and drones. Saudi Arabia launched a deadly war on Yemen in March 2015 with the aim of reinstating a former government friendly to Riyadh. Almost daily airstrikes have crippled civilian infrastructure and killed hundreds of thousands of people, many of them women and children. Riyadh has also imposed an all out air, sea and land blockade on its southern neighbor. The siege on Yemen has led to what the United Nations calls the worst humanitarian crisis in the world.