The U.N. envoy to Yemen sought on Tuesday to persuade Yemen's Houthi group to send representatives to peace talks in Kuwait as a shaky truce declared this month teetered near collapse, delegates said. An advisor to the U.N. delegation in Kuwait said the Houthis had been "very positive" until two days ago and were agreed with envoy Ismail Ould Cheikh Ahmed on almost everything. "They have since completely changed and this has caused a shock (to Ould Cheikh Ahmed)," the aide, who asked not to be identified, told Reuters. The envoy was now working with the Houthis and the government to iron out the problems. Houthi negotiators have stayed put in the capital Sanaa, demanding a ceasefire begun on April 10 be fully observed before traveling for the talks with envoys from Yemeni President Abd-Rabbu Mansour Hadi's government in Kuwait. The Houthis have also rejected a proposed agenda that stipulates they hand over heavy weapons and withdraw from areas they controls before a new government comprising all Yemeni forces is formed.

Any failure of the talks is likely to stoke intensified fighting between the Iran-allied Houthis and their ally, former President Ali Abdullah Saleh, on the one side and Hadi supporters, backed by a Saudi-led Arab coalition, on the other. Residents in Marib reported intensified fighting in the province east of Sanaa after fresh troop reinforcements loyal to Hadi arrived on Monday after training in Saudi Arabia. They also reported more fighting in Taiz in southwestern Yemen, despite the presence of ceasefire monitors, while Saudi-led warplanes flew over the Yemeni capital. Delegates representing Hadi's government have threatened to leave Kuwait, accusing the Houthis of trying to impose new conditions on the talks. Officials said delegates were meeting at the emir's palace to decide on their next move. The Houthis have observed a period of calm along the border with Saudi Arabia and exchanged prisoners with Riyadh, paving the way for Ould Cheikh Ahmed to draft a broad outline for the talks, which were due to start on Monday. The United Nations says the Yemen war has killed more than 6,200 people and displaced millions of people in the poorest country in the Arabian Peninsula.

The Houthis complain that Hadi's forces are trying to exploit the truce to try to make gains on the ground in several provinces, while war planes from the Saudi-led alliance continued to fly over areas held by the group. Teams of joint ceasefire monitors have been deployed in some areas, but the Houthis say they were still unable to curb continued violations of the truce.