Yemeni security forces fired tank and artillery shells at a restive central city on Friday, killing 11 people, including three women and a nine-year-old boy, a medical official said. Violence has escalated in the Arab world's poorest country following nearly nine months of a popular uprising seeking to oust longtime autocratic President Ali Abdullah Saleh. International diplomacy has failed to stop the crisis, with Saleh repeatedly refusing to sign a U.S.-backed proposal by Yemen's powerful Gulf neighbors. In the meantime, security across the country has broken down and clashes between government forces and army units who have joined the uprising regularly shake Yemeni cities. In the country's south, al-Qaida-linked militants have taken over entire towns, raising Western fears they'll use the area as a staging ground for international attacks.