WASHINGTON, Jan 6 (Reuters) - U.S. President-elect Barack Obama, breaking his silence about the Gaza war, expressed deep concern on Tuesday about the loss of civilian lives in Gaza and in Israel. Speaking after Israeli tank shells killed at least 40 Palestinians at a U.N. school where civilians had taken shelter, Obama told reporters "the loss of civilian life in Gaza and Israel is a source of deep concern for me." But Obama otherwise said he would adhere to his principle that only U.S. President George W. Bush would speak for American foreign policy at this time, but said he would have plenty more to say after his Jan. 20 inauguration. (Reporting by Steve Holland, Editing by Sandra Maler)