y raised the stakes in Yemen's nearly four-month-old political crisis and further undermined the stability of a deeply impoverished nation home to the most dangerous branch of al-Qaida and awash in weapons. President Ali Abdullah Saleh has clung to power despite daily protests, defections by key allies and intense pressure from the United States and powerful neighbors like Saudi Arabia to transfer his powers. His security forces have waged a brutal crackdown that has included sniper attacks on unarmed protesters, and he has several times pledged to step down under a deal with the opposition only to back out at the last minute. Beyond the capital, violence also roiled the southern city of Taiz, which has been a hotbed of anti-government protests since the early days of the uprising against President Saleh, who has ruled Yemen for 33 years. Soldiers backed by tanks and bulldozers moved in over the weekend, smashing a tent camp the protesters had held in a central square and destroying a field hospital that had been set up in anticipation of such an attack. A doctor who witnessed the attack said at least 20 people were killed on Monday. On Tuesday, the U.N.'s human rights office in Geneva said it received reports from Yemen that more than 50 people have been killed by pro-government forces in Taiz since Sunday. U.N. High Commissioner for Human Rights Navi Pillay cautioned that the reports "remain to be fully verified." Pillay said in a statement that "such reprehensible acts of violence and indiscriminate attacks on unarmed civilians by armed security officers must stop immediately." Three more people were killed in clashes in Taiz on Tuesday between security forces and demonstrators who had regrouped on several main streets, said activist Boushra al-Maqtali. Protesters set fire to tires and threw stones at the police, who fired rubber bullets, live ammunition and tear gas to disperse them, said another activist, Ghazi al-Samie. "The city is boiling," al-Samie said. "All shops have