"We call on Gulf Cooperation Council states to put pressure on the president to take all necessary measures to force him to sign the agreement," said Mohammed Qahtan, spokesman of the Common Forum, an alliance of parliamentary opposition groups.
"We are ready to sign anytime, but it's the president who is refusing."
Last week, GCC Secretary General Abdullatif al-Zayani travelled to Sanaa to invite members of the government and the opposition to sign the transition plan in Riyadh and to obtain the president's signature.
However, Zayani left empty-handed after Saleh, in power for 32 years, refused to sign.
Zayani on Wednesday said the GCC is "in contact with all Yemeni parties to reach an agreement over the procedures for signing the accord."
GCC members "are concerned with putting an end to bloodshed in Yemen and with maintaining its unity, security and stability," Qatari news agency QNA quoted Zayani as saying.
"The differences over the procedures concerning the signing of the agreement will be resolved through consultation within ministers of the GCC, its general secretariat and the Yemenis," he said.
On Sunday, GCC foreign ministers said Zayani would return to Sanaa to try again, but no date has yet been announced for his return as the Gulf mediators await a "signal" from Saleh to resolve the dispute.
The plan proposes the formation of a government of national unity, Saleh transferring power to his vice president and an end to the deadly protests rocking the impoverished Arabian peninsula nation since late January.
The president would submit his resignation to parliament within 30 days, to be followed two months later by a presidential election.
However, a defiant Saleh has publicly insisted on sticking to the constitution in any transfer of power, even though his ruling General People's Congress party has said it accepts the GCC plan.
At least 150 people have been killed in the protests that continue to rage across the impoverished country.