Unrest is intensifying in Yemen, where a battle rages in the capital city and troops have fired on protesters in the southern city of Taiz.

Reports Thursday said thousands of armed tribesmen are heading to the capital, Sana'a, to join the fight against government forces.

Government forces are reported to be shelling the positions of those loyal to tribal leader Sheik Sadeq al-Ahmar in the Sana'a neighborhood of Hasaba.

Officials say at least 41 people from both sides were killed the fighting Tuesday night and Wednesday.

In the southern city of Taiz, troops have fired on protesters who are calling for an end to the rule of President Ali Abdullah Saleh. At least 25 people have died in the clashes Taiz in the past few days.

Fighting has intensified in Yemen since the collapse of a cease-fire that was announced Friday.

In Hasaba, tribal fighters have seized a number of government ministries and buildings. Loyalist artillery fire has heavily damaged the compound of al-Ahmar, the strongest rival to President Ali Abdullah Saleh, and the government cut the area's electricity and water supplies.

Mr. Saleh's forces also used mortars and rocket-propelled grenades Wednesday to attack the headquarters of Brigadier General Ali Mohsen al-Ahmar, the country's most powerful military leader.

General Mohsen defected to the opposition in March. He is from the influential Hashid tribe, along with al-Ahmar, whose fighters last week joined a nearly four-month anti-government uprising to force Mr. Saleh from power.

The fighting has caused a number of countries to close or scale back their diplomatic missions. Kuwait said Wednesday it had evacuated its embassy staff, joining Italy, Qatar and Germany in suspending operations in Yemen. The U.S. last week moved most non-essential staff out of the country.