Showing no sign of warming to Syria, the Bush administration on Wednesday sharply rejected a charge of US colonialism by President Bashar Assad and demanded Syria cease its support for terror groups. Even as the Iraq Study Group, a bipartisan panel led by former Secretary of State James A. Baker III and former Rep. Lee H. Hamilton, weighs recommending a more conciliatory US foreign policy, the State Department struck back hard at Damascus. Among possible changes the panel is believed to be considering is a renewal of diplomatic dialogue with Syria, as well as Iran.