U.S. and Yemeni officials say President Ali Abdullah Saleh suffered injuries far more severe than previously reported in last week's attack on his presidential palace, raising doubts about his return to power — even as fighting in the south killed dozens of people.

Mr. Saleh was initially said to have received a shrapnel wound, and his vice president was quoted on Monday as saying the president would return to Yemen within days from Saudi Arabia where he is being treated.

But diplomatic sources said Tuesday that Mr. Saleh had burns over 40 percent of his body, including his face, neck and chest. He also is believed to have suffered a serious head injury.

Yemen's prime minister, his two deputies, the heads of the two houses of parliament and the head of the ruling party's legislative bloc were all evacuated to Saudi Arabia with severe injuries.

Meanwhile, the unrest that has engulfed the nation for months shows no signs of ending, despite the president's absence.

Fresh fighting erupted in Yemen's second-largest city, Taiz, where armed opposition tribesmen have occupied a large area in the center of the town. Witnesses heard explosions and gunfire as army tanks fired on the tribal fighters. Tuesday's violence in Taiz, the site of some of the biggest anti-Saleh protests since February, killed at least four people.

Government forces also clashed with Islamist militants in the southern city of Zinjibar, more than a week after hundreds of militants seized control of the town. The Defense Ministry said Tuesday that security forces had killed at least 30 militants in an effort by residents and government troops to retake the city.

Clashes had initially diminished in Mr. Saleh's absence, although opposition fighters say government snipers in the Yemeni capital, Sana'a, have killed at least three supporters of tribal leader Sheikh Sadiq al-Ahmar.

The fighting comes amid increased calls for Mr. Saleh to accept a peace deal put forward by the Gulf Cooperation Council that would end his nearly 33-year rule.

About 4,000 protesters in Sana'a gathered near Vice President Abd al-Rab Mansur Hadi's residence Tuesday, demanding the acting leader form a transitional council to create a new government and prevent Mr. Saleh from returning to power.

British Foreign Secretary William Hague Tuesday called for an immediate political transition in Yemen, urging Mr. Hadi to implement the GCC agreement.

Saudi officials say it is up to Mr. Saleh whether he goes home or not, but there have been suggestions the government in Riyadh may block his return and that the U.S. and European Union are also pushing for him to stay in Saudi Arabia.

Also Tuesday, an unidentified gunman trying to cross into Yemen from Saudi Arabia killed two Saudi border guards. Saudi Arabia's Interior Ministry says the gunman also was shot dead. The clash occurred at the Wadia border crossing, a conduit for drugs, weapons and people smuggling.

Nearly 400 people have been killed since a popular uprising against Mr. Saleh began in January.