SAN'A, 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 spells 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 extremists and opposition tribesmen in two southern cities 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-Qaeda's branch in Yemen - one of the terror network's most active branches, blamed for two attempted anti-U.S. attacks - will take advantage of the chaos to strengthen its base there.

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 San'a spoke with Vice President Abed Rabbo Mansour Hadi, who is acting president, to press the American view, State Department spokesman Mark Toner said in Washington.

Toner said he was unsure how long Saleh would undergo treatment in Saudi Arabia, or whether he still planned to return. But he said Yemen needed to move forward in the meantime.

Friday's attack on Saleh's palace compound came amid two weeks of battles in San'a 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 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 reignite fighting in the capital, which is only barely being contained by a Saudi-brokered cease-fire.

But three U.S. officials, speaking on condition of anonymity because of the situation's sensitivity, said Saleh had burns over 40 percent of his body and bleeding in his skull. Yemeni officials have said Saleh suffered heavy burns on his face, neck, and chest. One of the operations was to remove wood fragments embedded in his chest.

Yemeni officials have said a rocket hit a mosque in the 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 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.

The cause of the blast 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.