US forces dropped a pair of 2,000-pound bombs early yesterday morning on a bunker complex believed to be an insurgent training facility on the southern edge of this city, where the most dedicated and best trained rebel fighters are making a last stand.

The bombs shook the ground of the former insurgent stronghold and set off secondary explosions that went on for 45 minutes but could not be seen above ground, persuading officers of the Army's First Infantry Division that there were large stockpiles of weapons underground.