Yemen's Houthi rebels have accused the exiled government of Abd-Rabbu Mansour Hadi of attempting to impose its own agenda on the UN-brokered peace talks in Geneva.
Abdel-Malik al-Houthi, the group's leader, accused the government of using the United Nations and envoy Ismail Ould Cheikh Ahmed as a "tool".
"Leave to the United Nations some neutrality to continue its mission... Stop your continuous attempts to control its new envoy," said Houthi on Tuesday, pointing out that the government was "seeking to hamper any serious ... outcomes that could resolve the country's political situation. They want chaos".
Following initial meetings with the UN envoy, delegate Ghaleb Mutlak said that the rebels are trying to achieve a truce for the Muslim holy month of Ramadan, and that their delegation is willing to stay in Switzerland as long as it takes to end the bloodshed.
UN special envoy Ismail Ould Cheikh Ahmed said nothing will be settled unless the two sides can be persuaded to sit down together, but that just getting them both to Geneva was a "great achievement".
"We should not underestimate the significance of this event," he told reporters after meeting with the Houthi delegation.
"It is the important start towards the return to a political process. Let us be realistic, it will be a difficult path, but the important issue is that we start addressing the crisis."
It is unclear how long the talks - at least initially involving mediators shuttling between the parties, rather than face-to-face encounters - will last.
UN spokesman Ahmad Fawzi said the start of Ramadan later this week may affect whether the delegations stay in Geneva.
"It is a golden opportunity to try and resolve this crisis," Fawzi said. "Whether they will agree to extend their stay beyond the beginning of Ramadan is anybody's guess."
The delegation from Sanaa includes loyalists of Saleh and representatives of other political groups.
The Houthis, alongside troops loyal to former President Ali Abdullah Saleh, seized Sanaa in September and later expanded across the country, battling Hadi's supporters.
In March, an Arab coalition was formed to launch air strikes against the rebels as Hadi sought refuge in Saudi Arabia.
A Houthi delegation had held talks earlier this month with US diplomats in neighbouring Oman.