DUBAI/ADEN/GENEVA: U.N.-sponsored Yemeni peace talks were struggling amid disputes over releasing prisoners, sources close to the talks said Thursday, as local officials reported intensifying clashes and fresh airstrikes despite a cease-fire.

Saudi-backed President Abed Rabbou Mansour Hadi’s representatives are demanding that their foes, the Iran-allied Houthi movement, release several senior officials.

Peace talks began Tuesday away from television cameras in Switzerland to try to end nearly nine months of conflict that have killed almost 6,000 people and displaced millions.

The sources said that direct talks between the two sides have been suspended since Wednesday evening, after the Houthis rejected demands to free senior officials, including Defense Minister Mahmoud al-Subaihi and Hadi’s brother, Nasser.

Both Subaihi and Nasser Mansour Hadi, who was responsible for intelligence operations in southern Yemen have been held by the Houthis since March.

The Houthis say they are ready to free the prisoners once a permanent ceasefire is agreed, another source close to the talks told Reuters.

The sources said that instead of overseeing direct talks, U.N. special envoy to Yemen Ismail Ould Cheikh Ahmed was shuttling between the two sides trying to bridge differences.

Combat escalated between the warring sides Thursday as pro-Hadi fighters ejected the Houthis from the Al-Mas camp, their last military base in the central desert province of Marib after two days of heavy combat. According to local officials and tribesmen, 15 people were killed from both sides.

Loyalist forces stormed the base, accusing them of firing at pro-government troops, the sources said.

The rebels meanwhile accused the loyalists of attacking their positions in Marib.

Airstrikes by the Arab-led coalition also struck the northern Hajja province on the border with Saudi Arabia, residents said. They also said that gunboats struck Midi port, also in Hajja, near the Saudi border.

In Baida in central Yemen, a car bomb targeted a police station, a military source said, but it was unclear if there were casualties.

Both sides have been trading accusations over violating the ceasefire. On Wednesday, the coalition spokesman, Brig. Gen. Ahmed al-Assiri, accused the Houthis of committing some 150 violations since Tuesday and urged the United Nations to try to save the truce.

In a rare positive gesture Wednesday, the Houthis and fighters from the so-called Southern Resistance, who are allied with Hadi, exchanged hundreds of prisoners early on Thursday, following delays by local tribesmen angry at the exclusion of relatives from the deal, an official involved in the swap said.

Fikri al-Mutaili, a southern commander, said the Houthis freed 265 residents of the former south Yemen while the Southern Resistance freed 300 Houthis, including 40 teenagers.

The deal was brokered by local tribal leaders and had been expected to reflect positively on the peace talks in Switzerland.

A U.N. statement Thursday also said a large convoy carrying humanitarian aid had reached Taiz, and deliveries are expected to reach other cities, including Hajja and Saada, in coming days.

It said talks would continue over the next few days with a focus on achieving a “sustainable national cease-fire,” force withdrawals and the return of heavy weapons and control of public institutions to the state.