A cease-fire in Yemen's capital was at risk of unraveling Monday as regime supporters opened fire on opposition fighters in renewed clashes that killed at least six. The violence raises fears over the potentially explosive situation after the wounded President Ali Abdullah Saleh left the country, creating a deep power vacuum.

In his absence, the United States, Saudi Arabia and the Yemeni opposition were pushing to quickly lock Yemen into a transition that would ensure Saleh does not retain power, opposition officials said.

But so far Saleh seems determined to return and continue to wield power after he underwent surgery Sunday in neighboring Saudi Arabia for wounds suffered in a rocket attack on his compound.

"Saleh's health is improving greatly and he will return to the country in the coming days," Vice President Abed Rabbo Mansour Hadi, who is acting leader in the president's absence, said Monday.

If the president were to return, it would almost certainly re-ignite fighting that shaken the capital the past two weeks, between government troops and opposition tribesmen determined to oust Saleh. That could throw the country into even greater chaos _ and even outright civil war, if the opposition becomes convinced that the only way to remove him is through force.

The United States fears that al-Qaida's branch in Yemen could exploit the turmoil to strengthen its presence in the country, which it has already used as a base for plotting two attempted anti-U.S. attacks. A U.S. official said there were furious diplomatic efforts underway involving the Saudis, the U.S., the Yemenis and Gulf Arab nations _ a process he likened to "four-dimensional chess."

"We want to see a peaceful transition," the official said, speaking on condition of anonymity because he was not authorized to speak publicly on the matter. "We want the transition to begin immediately."

The focus is on reviving a U.S.-backed deal mediated earlier by the Gulf Cooperation Council, a grouping of Gulf Arab nations including Saudi Arabia. Under the deal, Saleh would step down, handing power to his vice president, a unity government would be formed and presidential elections held within two weeks. In the past three weeks, Saleh refused three times to sign the deal. As he was being evacuated for surgery over the weekend, he defied heavy Saudi pressure and refused to even sign a presidential decree formally handing over his power to Hadi, a sign he was intent on coming back.

"The Gulf initiative is the most appropriate way to solve the Yemen crisis," Abdullatif al-Zayani, a Saudi who heads the Gulf council and was the top mediator of the deal. "The GCC countries can activate it, and follow up on its implementation if all Yemeni parties declare their support for it."

With Saleh out of the country and power effectively in the vice president's hands, now is the time to push through the agreement, the Yemeni opposition says.

A Yemeni opposition leader, Abdullah Awabal, who met a day earlier with the U.S. ambassador in Sanaa, said the Saudis and the Americans and Europeans are all "in agreement to implement the initiative now. There can be no waiting."

But he said there appears to be divisions within Saleh's ruling party over whether to accept the deal in the president's absence. Deputy Information Minister Abdu al-Janabi on Monday insisted, "Nothing will happen without the approval of the president."

Saleh still has a powerful presence on the ground to back his hand: his sons and nephews, who command Yemen's strongest military units and who remain in the country. Their forces remained deployed around Sanaa on Monday, locked in a tense standoff with tribal fighters. Saleh's family may have the most to lose in any deal, since many in the opposition demand that their lock on top government and security positions be broken.

Amid the uncertainty, the cease-fire brokered by Saudi Arabia as Saleh left for treatment appeared shaky.

Gunmen _ apparently pro-Saleh forces _ attacked tribal fighters loyal to Sheik Sadeq al-Ahmar on Monday, killing three tribesmen, al-Ahmar's office said. The shooting took place in the Sanaa district of Hassaba, where al-Ahmar's residence is located and which has been the epicenter of the past two weeks of fighting.

Late Sunday, pro-government gunmen opened fire on a checkpoint manned by a military unit that defected and joined the opposition, an officer from the unit said. In the clash, two of the attackers and one of the unit's soldiers were killed, the officer said, speaking on condition of anonymity because he was not authorized to talk to the press.

Hassaba remained tense, with government forces dug in despite promises under the cease-fire that they would pull back from their positions. Residents trying to return to their homes in the neighborhood were forced back by snipers firing from rooftops, another pro-opposition military officer said. While unable to enter the district, an Associated Press reporter who reached the edges could see broken electricity pylons and shops and buildings pockmarked by mortar shrapnel.

In the Saudi capital, Riyadh, Saleh underwent successful surgery on his chest to remove jagged pieces of wood that splintered from a mosque pulpit when his compound was hit by rockets on Friday. He also had a procedure on a nerve in his neck and plastic surgery to treat burns.

A medical official at the Saudi Health Ministry said Saleh was awake, spoke to his doctors and is in good health.He is expected to be released from the ICU soon, the official said, on condition of anonymity because he is not authorized to talk to the media.

The stunning rocket attack, which the government first blamed on tribal fighters and later on al-Qaida, killed 11 bodyguards and seriously injured five senior officials worshipping at Saleh's side.

A crucial question is whether Saleh's Saudi hosts want him to return. The Saudis backed Saleh and cooperated in confronting al-Qaida and other threats, but they are now among those pressing him to give up power as part of a negotiated deal. Saudi Arabia is eager to contain the unrest on its doorstep.

Yemen's unrest began as a peaceful protest movement that the government at times used brutal force to suppress, killing at least 166 people, according to Human Rights Watch. It transformed in the past two weeks into armed conflict after the president's forces attacked the home of a key tribal leader and one-time ally who threw his support behind the uprising. The fighting turned the streets of the capital into a war zone.

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Associated Press writers Sarah El Deeb in Cairo, Matthew Lee in Washington and Abdullah al-Shihri in Riyadh contributed to this report.