WASHINGTON - The United States on Saturday expressed deep concern about events in Yemen and reiterated its call for President Ali Abdullah Saleh, who returned to the nation on Friday, to make "a full transfer of power without delay." Mortar shells and sniper fire struck an opposition protest camp in the capital Sanaa on Saturday as General Ali Mohsen, Saleh's main military rival in the Arabian Peninsula nation, said the president's return after a three-month absence could spark civil war. State Department spokeswoman Victoria Nuland said in a statement that the United States was "expressing deep concern about the current situation in Yemen."