Yemeni troops opened fire Sunday on a large group of anti-government protesters in the capital, Sana'a, killing one person.

Sunday's shooting is the latest violence in Yemen since President Ali Abdullah Saleh unexpectedly returned to the country Friday after a 3-month stay in neighboring Saudi Arabia, recuperating from a June assassination attempt.

On Saturday, clashes between forces loyal to the president and student protesters backed by military forces in Sana'a, left at least 40 people dead. Government forces also attacked a student protest camp, causing many casualties.

A spokesman for opposition General Ali Mohsen al Ahmar said at least 11 soldiers loyal to him were killed after their position came under fire.

Forces loyal to President Saleh reportedly have targeted the homes of tribal leader Sheikh Sadek al Ahmar and his supporters, who no longer backs Mr. Saleh.

Al Arabiya TV said the 6-nation Gulf Cooperation Council has appealed to all sides to observe a cease-fire and “stop using heavy artillery against unarmed protesters.”

The U.S. State Department has expressed deep concern about the situation in Yemen. A statement Saturday urged all parties to cease violence and exercise maximum restraint. It called on President Saleh to initiate a full transfer of power and arrange for presidential elections to be held before the end of the year.