The top US diplomat for the Middle East will step down from his post and move to the private sector at the end of the week, as President-elect Barack Obama's transition team weighs possible successors, State Department officials said Wednesday. David Welch, a career diplomat and assistant secretary of state for Near Eastern affairs, has told his staff his last day on the job will be Friday, the officials said. The officials spoke on condition of anonymity before a formal announcement by Secretary of State Condoleezza Rice, expected Thursday. Welch is the first of the department's assistant secretaries to leave since Obama's election last month. Assistant secretaries of state, like ambassadors, are political appointees and are required to submit resignation letters before a change in the administration. Some, however, are asked to stay on by the incoming administration.