If the entire premises are destroyed by fire or other casualty, this lease will immediately terminate. In case of partial destruction or damage, so as to render the premises untenantable, as determined by the Government, the Government may terminate the lease by giving written notice to the Lessor within 15 calendar days after such determination; if so terminated, no rent will accrue to the Lessor after such partial destruction or damage; and if not so terminated, the rent will be reduced proportionately by supplemental agreement hereto effective from the date of such partial destruction or damage. Nothing in this lease shall be construed as relieving Lessor from liability for damage to or destruction of property of the United States of America caused by the willful or negligent act or omission of Lessor.