The biggest U.S.-led ground offensive of the five-month Afghan war entered a second day on Sunday with B-52 planes again bombing thousands of Taliban and al Qaeda forces in mountain bunkers in east Afghanistan. U.S. warplanes dropped two recently developed "thermobaric" 2,000 pound (907 kg) bombs, which create a high-pressure blast that drives air out of a cave and potentially suffocates those inside, U.S. defense officials said. Afghan troop reinforcements and U.S. advisers, repelled by the diehard fighters in the opening assault on the snow-covered mountain caves, gathered near Gardez, capital of Paktian Province, about 150 km (95 miles) east of the capital of Kabul, to prepare for a new attack.