The head of a coalition of Gulf countries seeking to broker an end to Yemen's political crisis gave up on Wednesday and left the country, opposition and government leaders said.

Yemen is reeling from three months of massive street protests demanding the ouster of President Ali Abdullah Saleh after more than three decades in power.

The Gulf Cooperation Council sought to mediate a deal for Saleh to leave power in exchange for immunity from prosecution. Saleh snubbed the deal last month, prompting a visit from the coalition's head, Abdul-Latif al-Zayyani, to try to break the impasse.

But al-Zayyani, who is from Bahrain, ended his five-day visit Wednesday without closing the deal, leaving each side blaming the other for its failure.

In Washington, the White House said that John Brennan, who is an assistant to U.S. President Barack Obama, had called Saleh on Wednesday urging him to accept the GCC-brokered plan. He called it "the best path forward for Yemen to become a more secure, unified and prosperous nation."

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This represents a change in the U.S. stance toward Saleh, who was considered an ally in fighting al-Qaida's active Yemeni branch.

Opposition spokesman Mohammed al-Sabri said al-Zayyani told the opposition he was leaving because he couldn't get Saleh to sign.

"He said that since they were not able to reach an agreement, he was leaving Sanaa and would not come back," al-Sabri said.

Al-Sabri said Saleh had repeatedly tried to amend the deal by adding conditions that the opposition rejected.

Ruling party official Yasser al-Yemani confirmed al-Zayyani's departure, adding that Saleh refused to sign until the sit-ins across the country had ended.

"Saleh will not leave power as long as the security situation remains unstable," he said.

The mass protests have posed an unprecedented challenge to Saleh's rule. Several top military commanders and ruling party officials have defected to the opposition, while a crackdown by government forces has reportedly killed more than 150 people.

Yemen, Saudi Arabia's southern neighbor, faced crises even before the protests. It is plagued with widespread corruption, a weak central government, a Shiite rebellion in the north, a secessionist movement in the south and an active branch of al-Qaida in its weakly governed provinces.

The GCC nations behind the mediation effort were Bahrain, Kuwait, Oman, Saudi Arabia and the United Arab Emirates.