(Reuters) - Susan Rice is facing her first key test as President Barack Obama's national security adviser as she helps lead a White House effort to convince a skeptical Congress that the United States must respond to Syria's apparent use of chemical weapons.

Rice has made an abrupt transition from a high-profile and sometimes combative U.S. ambassador, who scolded the U.N. Security Council for failing to agree on Syria sanctions, to a behind-the-scenes power player at the White House.