SANAA — U.N.-sponsored Yemen peace talks are struggling amid disputes over releasing prisoners, sources close to the talks said on Thursday, as local officials reported intensifying clashes and fresh air strikes despite a ceasefire.

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

Combat escalated between the warring sides on Thursday as Hadi loyalists seized a key military base from Houthi fighters in the central city of Marib, local officials and tribesmen said, where 15 people were killed from both sides.

Planes and gunboats from a Saudi-led military coalition also bombarded targets in northern Yemen, local residents said.

But the U.N. special envoy for Yemen, Ismail Ould Cheikh Ahmed, said the third day of peace talks had produced “a major step forward” on the humanitarian front – agreement on aid for the war-torn city of Taiz – that would pave the way to further agreements on aid and other issues in coming days.

Yemen plunged into civil war last year when the Houthis seized the capital Sanaa and marched south, triggering the mainly Gulf Arab military intervention in March.

Peace talks began on 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 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, Ould Cheikh Ahmed was shuttling between the two sides trying to bridge differences.

The U.N. statement said the talks would continue in the next few days to try and define a clear way forward, with a special focus on areas such as developing a sustainable national ceasefire, withdrawal of forces and the release of prisoners.

Heavy weapons would also be returned to the state, which would also take control over public institutions, it said.