By Roberta Rampton and Arshad Mohammed HANOVER, Germany/WASHINGTON (Reuters) - President Barack Obama will announce on Monday he plans to send as many as 250 additional U.S. troops to Syria, a sharp increase in the American presence working with local Syrian forces fighting Islamic State militants, U.S. officials said. The deployment, which will increase U.S. forces in Syria to about 300, aims to accelerate recent gains against Islamic State and appears to reflect growing confidence in the ability of U.S.-backed forces inside Syria and Iraq to claw back territory from the hardline Sunni Islamist group.