Military sources report Najef standoff nearing climax.US Marine/Iraqi National Guard cordons thrown up around Shiite City-one external, the second enclosing the Old City where fighting rages around Imam Ali Mosque.Reports Sadr had booby trapped shrine and threatens to blow it up with 2,000 militiamen inside if US-Iraqi forces storm in.US F-16 jets level Old City House thought occupied by Sadr.US Generals seek fast end to showdown before general conflagration sparks.