The talks were initially scheduled to start on Monday, and any further delay could dash hopes of ending Yemen's war after the government delegation threatened to pull out if meetings did not begin immediately.
"According to the latest information, the rebel delegation should arrive in Kuwait by the end of the day," said one diplomat close to the talks.
"As a result, the talks could be delayed further until Friday," another diplomat said.
Yesterday, UN spokesman Stephane Dujarric said negotiations would begin in Kuwait today.
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There was still no word today from the rebels on their expected time of arrival.
Representatives of the Iran-backed Huthi insurgents and their allies left Sanaa yesterday for Oman and they are expected to continue on to Kuwait.
But they were still in Oman today morning, according to diplomats.
In Yemen itself, fighting wore on on several fronts, military sources said, as each side blamed the other for truce breaches.
Pro-government military sources reported heavy fighting in Nahm, northeast of Sanaa, as well as sporadic clashes in the northern Jawf province, Taez in the southwest, and central Baida province.
The rebels meanwhile claimed on their sababnews.Net website that a Saudi-led coalition fighting them in support of the Yemeni government carried out two air strikes on Nahm and flew sorties over Sanaa, Marib, and Jawf.
The talks are the most important attempt yet to resolve Yemen's devastating conflict, which the UN says has killed more than 6,400 people and forced almost 2.8 million forced from their homes.