Security forces in Yemen have shot dead at least 20 anti-government protesters in the southern city of Taiz. Reports say the soldiers moved in to end a four-month-old sit-in in the city. Meanwhile, four Yemeni soldiers have been killed in an apparent ambush near the city of Zinjibar, reportedly held by al-Qaeda fighters. President Ali Abdullah Saleh has refused to step down despite months of protests and strong opposition from a key tribe.