Shelling and gunfire echoed across Sana for a third day, including in an area near the airport, and white flashes lighted the night sky. Residents said that tribesmen who are clashing with government forces had seized Yemen's state-run news agency. In the preceding 48 hours, anti-Saleh fighters captured the interior and trade ministries.

The continuing violence has claimed at least 40 lives in the last three days, though some reports put the toll as high as 60. It has raised fear of a wider battle between Saleh and the heads of his own Hashid tribe, including former ally Sadiq Ahmar, whose house was attacked Monday by Saleh's forces.

"What happened was a provocative act to drag us into civil war, but it is limited to the Ahmar sons," Saleh said Wednesday in a news conference. "They bear responsibility for shedding the blood of innocent civilians. Until this second, they are attacking the Interior Ministry. But we don't want to widen the confrontation."

Saleh has fought off multiple threats to his leadership in his 32 years in office. In 1994, government troops crushed a civil war with southern separatists, who sought a return to the region's pre-1990 status as an independent socialist state. Northern Houthi rebels, who want autonomy in the province of Saada, have fought six wars against the government since 2004.

But the recent unrest, which began in January as a protest movement, has created a common goal for the southern separatists and northern Houthis. Both groups have pledged their support for pro-democracy demonstrators calling for Saleh's immediate resignation.

The major concern of Saleh's Western supporters is that continued instability may strengthen Al Qaeda in the Arabian Peninsula. The United States provided $150 million in military aid to Saleh in 2010 to battle the Islamic militant group.

Yemen's military and its U.S.-trained counter-terrorism unit have withdrawn from large parts of the country in recent months as Saleh has focused on crushing dissent in the capital.

Islamic militants claim to have taken control of several towns in the vulnerable southern provinces known to harbor Al Qaeda terrorists, leading online militant forums to rename one province the Islamic Emirate of Abyan.

In a recent seminar of Al Qaeda experts held in Sana, analysts discussed the long-term goals of the terrorist network in Yemen in light of recent events.

"What is happening now, with revolutions in Tunisia, Egypt and the uprising here in Yemen, will weaken Al Qaeda," said Dr. Ahmed Al Daghshi, a professor at Sana University. "These revolutions have pushed for regime change by peaceful means, not by violence as Al Qaeda has done."

Western concerns were on full display Wednesday as Obama called on Saleh "to move immediately on his commitment to transfer power."

Obama has so far refrained from directly calling for Saleh's resignation. Speaking at a news conference in London with British Prime Minister David Cameron, the president reiterated U.S. support for a deal brokered by an alliance of Yemen's neighbors. The proposal, which would have seen Saleh step down within 30 days, collapsed Sunday as the Yemeni leader reneged on his commitment to sign the pact.

**Craig is a special correspondent.

*Times staff writer Ned Parker in Baghdad contributed to this report.