Whenever any person is delivered by any foreign government to an agent of the United States, for the purpose of being brought within the United States and tried for any offense of which he is duly accused, the President shall have power to take all necessary measures for the transportation and safekeeping of such accused person, and for his security against lawless violence, until the final conclusion of his trial for the offenses specified in the warrant of extradition, and until his final discharge from custody or imprisonment for or on account of such offenses, and for a reasonable time thereafter, and may employ such portion of the land or naval forces of the United States, or of the militia thereof, as may be necessary for the safe-keeping and protection of the accused.