DUBAI (Reuters) - Yemen may have little chance of averting a tribal civil war as heavy fighting spreads in the capital unless President Ali Abdullah Saleh quickly resigns.
But Saleh, a stubborn political survivor, has likely already decided to fight to keep power in the strategic state where Gulf and Western allies are concerned that anarchy could give the strong Yemen-based branch of al Qaeda more room to operate.
Saudi Arabia, fearing instability in a neighbor located on a shipping lane through which 3 million barrels of oil pass daily, may decide to press Saleh harder to leave. But this is unlikely to be enough, barring a coup or military intervention.
"It seems that we have crossed a rubicon here," Dubai-based security analyst Theodore Karasik said, adding that the chances for a democratic or peaceful transition of power were now slim.
"It is going to look like Libya, and now it is becoming ... like Libya. And I think that is important for the international community to recognize."
Fighting between heavily armed Saleh loyalists and opposition groups has killed scores of Yemenis since it erupted in Sanaa on Monday, a day after Saleh refused for the third time to sign a pact mediated by Gulf neighbors that would have ended his 33-year tenure in the fractious country within a month.
Absent from these street battles are the tens of thousands of protesters who continue holding peaceful daily rallies demanding Saleh's ouster and a transition to a democracy. It is unclear how they will respond to the escalating violence.
The clashes in the sandbagged streets surrounding the Sanaa mansion of tribal leader Sadiq al-Ahmar are the most sustained yet since youth-led protesters set up tent camps in cities across Yemen in February to heap pressure on Saleh to bow out.
Tribal allegiances are the most powerful element of Yemen's volatile social fabric and the fighting already appears to be playing out along tribal lines. Ahmar, head of the Hashed tribal federation to which Saleh's Sanhan tribe also belongs, called on all Yemeni tribes to back him against the president.
"He will leave this country barefoot," Ahmar said of the man who has maintained authoritarian rule over Yemen for 33 years.
Saleh also ramped up his rhetoric, accusing Ahmar and his brothers -- sons of a tribal patriarch who had been his ally -- of trying to drag Yemen into open war, saying they had "made the wrong decision" in confronting the state.
A key army commander who has backed protesters inspired by revolts that toppled the leaders of Egypt and Tunisia called on the military to defy Saleh. General Ali Mohsen called the president a "madman who is thirsty for more bloodshed."
"It's not yet a civil war. If this can be contained in a couple of days then things will be fine. If things are not fine, things will escalate into a civil war," Yemeni political analyst Abdul-Ghani al-Iryani said.
"If (General) Ali Mohsen al-Ahmar comes into the fray, the entire military will get involved... It will be an all-out war."
In a sign that ordinary civilians were expecting the worst, Yemenis streamed out of the capital, bags strapped to the roofs of their cars. Fighting erupted near the airport, and tribal gunmen loyal to Ahmar seized control of several ministries.
The Gulf Cooperation Council, which spearheaded mediation efforts, suspended its initiative after the head of the six-nation bloc was trapped for hours in the United Arab Emirates embassy on Sunday by pro-Saleh gunmen intent on scuttling the deal. The president later refused to sign.
Yemen's Western and Gulf allies, fearing a failed state on the doorstep of the world's top oil exporter, desperately want to see a peaceful transition of power and increasingly see Saleh as a liability as his domestic supporters desert him.
The United States, which has leaned on Saleh in its fight against al Qaeda, and France stepped up their calls on Thursday for Saleh to go, blaming the increased bloodshed on his backpedalling from the transition accord.
Saudi Arabia, which also has longstanding and strong ties with Yemeni tribes, is likely to try to apply another round of pressure on Saleh to step aside to avert disaster in a country where half of the 23 million people own a gun.
"The only thing Saudi Arabia has left is to throw its psychological or moral support or for someone to come out and openly ask him to leave," said prominent Saudi journalist Jamal Khashoggi, adding that such a request could ask him to step aside "in the name of Islam, friendship and Arabism."
Saudi Arabia is in a tough position. It wants to discourage revolts in the Arabian Peninsula that have also hit Bahrain and Oman and it fears could eventually spread to its own territory. But it doesn't want its own security threatened by instability that it believes could erupt if the Yemeni standoff festers.
Most analysts agreed Saudi pressure would not extend to military intervention, although there is a precedent for such action. Riyadh was drawn into Yemen's civil war with northern Shi'ite rebels in 2009, when it sided with the government.
Saudi Arabia sent troops to Bahrain in March to help the fellow Sunni Muslim monarchy suppress mostly Shi'ite protesters.
Karasik suggested Riyadh could step in at the request of Yemeni tribes with whom it has longstanding ties, although there was no sign this was currently on the table. Others said the Saudis might refer the matter to the Arab League, although that would be unlikely to result in binding action.
"You need the Arab League to take a position ... But the Saudis will need to lead the effort here because they have influence and allies inside Yemen," said Khaled al-Dakhil, a Saudi politics professor.
Additional reporting by Nour Merza in Dubai and Jason Benham in Riyadh; Writing by Cynthia Johnston; editing by Mark Heinrich