The United States expresses deep concern about events in Yemen and reiterated its call for President Ali Abdullah Saleh, who returned to the country 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, President Ali Abdullah 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." "We urge all parties to cease violence and exercise maximum restraint," Nuland said. "Too many Yemenis have lost their lives and each day that passes without a peaceful and orderly transition is another day that the Yemeni people are forced to live in an unstable environment that threatens their security and livelihood," she added. Yemen has been rocked since January by protests demanding Saleh's ouster after 33 years in power. President Barack Obama's administration has called for Saleh to hand over power and arrange for a presidential election by year-end. "We again urge President Saleh to initiate a full transfer of power without delay and arrange for presidential elections to be held before the end of the year within the framework of the GCC (Gulf Cooperation Council) initiative," Nuland said. "The Yemeni people have suffered enough and deserve a path toward a unified, stable, secure and democratic Yemen," she added. US officials have also expressed ongoing concern about the presence of al Qaeda militants in Yemen.