"We will meet with the Gulf Cooperation Council ministers in Riyadh to ask for clarification on their initiative, especially on the departure of President Saleh," Yassin Saeed Noman, head of the opposition Common Forum said.
Saleh has since January faced protests calling for his departure, in which more than 125 people have been killed.
An opposition delegation -- led by Mohammed Salem Bassandawa, a former foreign minister, and also including Abdelwahab al-Ansi of the Islamist Al-Islah party -- was to travel to Riyadh later on Sunday, Noman said.
The Common Forum has made Saleh's departure a prerequisite for any political settlement, while the GCC's bid to resolve the crisis calls for him to hand over power to his vice president.
Saleh's office has said in response to the mediation bid of the six-nation GCC he has "no reservation about transferring power peacefully and smoothly within the framework of the constitution."
US and European diplomats have over the past week been working to bring the opposition and Saleh's camp together, a Western diplomat told AFP.
"The Common Forum has obtained American and European assurances on the success of the GCC initiative, especially for a rapid departure of President Saleh," said the diplomat, who spoke on condition of anonymity.
The opposition has for the time being ruled out dialogue with the regime, and on Saturday threatened not to send its delegation to Riyadh if government representatives were also in the Saudi capital.
A source close to the government said "no government representative will go today to Riyadh."
The GCC foreign ministers, holding their third such meeting in two weeks, were to "discuss the Gulf initiative concerning the situation in Yemen," said the Saudi state news agency SPA late Saturday.
After their last meeting on April 10, GCC ministers announced a proposal for Saleh to transfer power to his vice president, and for the formation of an opposition-led unity government ahead of fresh elections.