US warplanes unleashed one of the most intense airstrikes of the Iraq war, dropping 40,000 pounds (18,145 kilograms) of explosives in a thunderous 10-minute onslaught on suspected al-Qaida in Iraq safe havens in Sunni farmlands south of Baghdad. The mighty barrage on Thursday - recalling the Pentagon's "shock and awe" raids during the 2003 invasion - appeared to mark a significant escalation in a countrywide offensive launched this week to try to cripple remaining insurgent strongholds. But it also fits into the endgame strategy of last year's US troop buildup, which seeks to regain control of Baghdad and surrounding areas as a buffer zone for the capital. US commanders are now attempting to subdue the last insurgent footholds around Baghdad before the Pentagon faces a possible reduction in troop strength. Some of the additional 30,000 troops have been pulled out and the remainder are expected to depart by June, military officials have told The Associated Press. With insurgents still holding pockets south of the capital and in districts to the north and far north - including the key northern city of Mosul - the military apparently wants to take the remaining four months or so to use the expanded military muscle against al-Qaida.