SANAA, Yemen -- President Ali Abdullah Saleh was burned over 40 percent of his body and suffered bleeding in the brain from last weekend's attack on his palace, U.S. officials said Tuesday, indicating his wounds were worse than initially reported. The revelation casts doubts on a quick return to Yemen and spell a deepening power vacuum.

In the wake of Saleh's evacuation to Saudi Arabia for treatment, Yemen's violence escalated, with government troops battling Islamic militants and opposition tribesmen in two southern cities on Tuesday. The military said it killed 30 militants who were among a group that took over the city of Zinjibar last week amid the country's turmoil.

The United States fears that al-Qaida's branch in Yemen -- one of the terror network's most active, blamed for two attempted anti-U.S. attacks -- will take advantage of the chaos to strengthen its base in the country.

Washington and Saudi Arabia are pushing Yemeni officials to seize the opportunity of Saleh's evacuation to immediately begin a transfer of power and formation of a new government. The U.S. ambassador in Sanaa spoke with Vice President Abed Rabbo Mansour Hadi, who is acting president, to press the American view, State Department spokesman Mark Toner told reporters in Washington.

Toner said he wasn't sure how long Saleh would undergo treatment in Saudi Arabia, or whether he still planned on returning. But he said Yemen needed to move forward in the meantime.

"We need to see all sides moving forward on a constructive basis," he said.

Friday's attack on Saleh's palace compound came amid two weeks of battles in Sanaa between government forces and opposition tribesmen determined to drive him from power. The fighting pushed the impoverished country closer to civil war after four months of street protests by hundreds of thousands of Yemenis failed to oust Saleh, who has been in power for nearly 33 years.

On Monday, Hadi said Saleh, in his late 60s, was improving after a series of operations in Saudi Arabia and would return home "within days." If Saleh were to return, it would almost certainly re-ignite the fighting in the capital, which is only barely being contained by a Saudi-brokered cease-fire.

But the revelations by U.S. officials suggested Saleh was in no condition to return soon. Three officials said Saleh, in his late 60s, had burns over 40 percent of his body and bleeding in his skull. They spoke on condition of anonymity because of the sensitivity of the situation. Yemeni officials have said Saleh suffered heavy burns on his face, neck and chest. One of the operations in Saudi Arabia was to remove wood fragments embedded in his chest.

Yemeni officials have said a rocket hit a mosque in the presidential palace compound where Saleh and his senior leadership and several hundred others were praying. At least 11 guards were killed and more than 150 people wounded.

The strike was a devastating blow to Saleh's top circles of power.

The prime minister, his two deputies, the heads of the two houses of parliament and the head of the ruling party bloc in parliament were all evacuated to Saudi Arabia with severe injuries. One deputy prime minister, Rashad al-Alimi -- who is Saleh's most important security adviser -- was still unconscious when evacuated and his condition in Saudi Arabia is not known. The other deputy, Sadeq Abu Ras, lost a leg. The governor of Sanaa, Numan Duweid, suffered a severed leg and hand and was in a coma, Yemeni officials said.

The cause of the blast, however, remained unclear. Three U.S. officials and one former U.S. official said intelligence reports suggest the explosion was from a bomb planted in the mosque. The officials spoke on condition of anonymity to discuss the intelligence. Yemen's Deputy Information Minister Abdu al-Janadi said the attack was still under investigation.

Footage from the mosque after the attack, aired on state TV, seemed to have indications of a rocket attack, however, with a hole punched through the front wall -- near where Saleh and his officials would have been standing at the head of the congregation -- and no visible bomb crater amid the scattered wreckage of the room.

The uncertainty amid Saleh's absence with no sign of an imminent power transfer raises fears of escalating chaos.

The cease-fire has held shakily in the capital, but fighting continued in Yemen's second largest city, Taiz, which has been the scene of some of the biggest anti-Saleh protests since February -- and scene of some of the fiercest crackdowns. Tribal fighters entered the city late last week and attacked government troops, apparently to protect protesters or seek revenge for deaths in the crackdowns.

On Tuesday, tribesmen and troops clashed near Taiz's presidential palace. A shell fired by a tank near the palace landed in a nearby residential area, killing four people, including three children.

The situation in Zinjabar also hikes worries over rising power of Islamic militants, who overwhelmed the city late last month. The Yemeni government claims the militants are connected to al-Qaida. But their identity remains unclear. There are numerous armed Islamic militants in Yemen, most of them not directly members of the terror network, and many of them sometime-allies of Saleh's government. Saleh, for example, used armed extremists to fight secessionists in the south in a 1994 civil war and in more recent uprisings.

Government warplanes bombed areas around Zinjibar on Tuesday, according to witnesses and military officials.

The Defense Ministry said its forces carried out "cleansing operations" in Zinjibar and its outskirts late Monday and Tuesday, killing more than 30 militants. Among the dead, it said, was an al-Qaida figure, Hassan al-Aqili, who was a commander for the terror group in the neighboring province of Marib and was accused last year of killing a senior army commander there.

Amid the fighting, dozens of militants attacked an army position outside Zinjibar, prompting a gunfight that left nine soldiers and six of the attackers dead, according to the military officials, who spoke on condition of anonymity because they weren't authorized to release the information. The army regained control of the post after hours of fighting.

In a separate incident, Saudi Arabia said its border guards killed a Yemeni gunman who opened fire while trying to cross into Yemen in a jeep at a crossing near Najran, 60 miles (100 kilometers) inland from the Red Sea, early Tuesday. The Saudi statement said two guards were killed. No further details about the gunman were given. Infiltration in both directions along the 800-mile (1,300-kilometer) desert border is common.


Klapper reported from Washington. AP correspondents Ben Hubbard in Cairo and Pauline Jelinek, Lolita Baldor and Kimberly Dozier in Washington contributed to this report.