SANAA (AFP) - Yemen's political rivals failed Wednesday to sign a Gulf-brokered plan under which veteran President Ali Abdullah Saleh would leave office as demanded in almost four months of popular protests. Abdullatif al-Zayani, the head of the Gulf Cooperation Council mediating the crisis, flew out of Sanaa as sources close to the negotiations between the regime and opposition said the two sides again failed to ink the power transfer accord. The accord would see Saleh quit office within 30 days, in return for immunity from prosecution, before a government of national unity is formed and elections for a new president held after two months. Washington issued a fresh plea Wednesday for the deal to be signed when Yemen's opposition expressed doubts about it after an aide to Saleh, who has ruled the country since 1978, said the signing would happen on Wednesday. US President Barack Obama's aide John Brennan called Saleh to urge him to sign and implement the agreement "so that Yemen is able to move forward immediately with its political transition," a White House statement said. An aide to the embattled Yemeni president said the nation's political rivals had agreed to sign the plan brokered by the region's six-nation Gulf Cooperation Council on Wednesday. But parliamentary opposition spokesman Mohammed Qahtan said Saleh had backed out of signing the accord. "We came to an agreement late Tuesday but this morning they changed their minds," Qahtan told AFP, adding Saleh and his partisans "refuse" to sign it this way. According to the US statement, Brennan "noted that this transfer of power represents the best path forward for Yemen to become a more secure, unified, and prosperous nation and for the Yemeni people to realize their aspirations for peace and political reform." He also "affirmed the commitment of the United States to stand with the Yemeni government and people as they implement this historic agreement, foster economic development, and combat the security threat from Al-Qaeda in the Arabian Peninsula." Brennan "reiterated that all parties must refrain from violence and proceed with the transition in a peaceful and orderly manner," the statement said. On the ground, most of Yemen's cities observed a complete strike on Wednesday morning as police fired shots into the the air in the town of Huta, in southern Lahij province, and in the Red Sea city of Al-Hudaydah, witnesses said. Massive protests took place in Taez and Ibb, south of Sanaa, and Al-Hudaydah, witnesses said. In Huta, where protesters blocked roads, residents said police fired into the air. Yemen's southern cities of Aden, Lahij and Shabwa also went on strike, according to residents there. The impoverished but strategic Arabian Peninsula country has been gripped by protests since late January calling for the ouster of Saleh. Security forces launched a deadly crackdown the protests, leaving at least 180 people dead, according to a toll compiled from reports by activists and medics. Besides an Al-Qaeda resurgence, Yemen is battling a secessionist movement in the south and a Shia rebellion in the north. Saleh insists that, under the constitution, he should serve out his current term of office, which expires in 2013. Last Thursday, however, Washington called on him to sign the deal "now." For weeks, the agreement has been held up by Saleh refusing to sign in his capacity as president. He has insisted on endorsing the deal only as leader of the ruling General People's Congress, contrary to the opposition's demands. Talks are ongoing, said Qahtan, adding the opposition would meet Abdullatif al-Zayani, the secretary general of the Gulf Cooperation Council, later on Wednesday. On Tuesday, the spokesman for Saleh's party, Tareq al-Shami, had told AFP: "We have discussed with ... Zayani the mechanism to implement a plan to end the crisis." "This plan needs a time frame to implement it," said Shami. On Friday, Qahtan declared the Gulf initiative was "dead" following the pullout of Qatar, whose prime minister had angered Sanaa by saying Saleh should go. But Zayani returned to Sanaa on Saturday in an attempt to convince both sides to sign the initiative.