Clashes between forces loyal to Yemeni President Ali Abdullah Saleh and student protesters backed by military forces in the capital, Sana'a, left at least 40 people dead Saturday, a day after the president unexpectedly returned to the country and called for a truce.
Government forces attacked a student protest camp overnight, 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 targeted the homes of tribal leader Sheikh Sadek al Ahmar and his supporters, who no longer back Saleh.
Al Arabiya TV said the six-nation Gulf Cooperation Council appealed to "all sides to observe a ceasefire" and to "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.
Saleh called for an end to the fighting Friday, shortly after his return from Saudi Arabia where he was recovering from injuries sustained in a June attack on his presidential compound in Sana'a.