Yemen President Ali Abdullah Saleh was burned on more than 40% of his body and suffered bleeding in the brain from last weekend's attack on his palace, US officials said, 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 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-US 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 US ambassador in Sanaa spoke with Vice President Abed Rabbo Mansour Hadi, who is acting president, to press the American view, US State Department spokesman Mark Toner told reporters in Washington.
Mr Toner said he was not 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.