SANAA, Yemen — Hundreds of Islamic militants solidified their control over a town in southern Yemen Sunday after seizing tanks in clashes with fleeing soldiers, military officials said. Opponents of the country’s embattled president, Ali Abdullah Saleh, accused him of allowing the militants to freely seize Zinjibar in a bid to stoke Western fears that al-Qaida would take over the country if he responded to demands by demonstrators to step down. Three months of mass street protests calling for Saleh’s ouster have posed an unprecedented threat to his 33-year rule.