It was countered by the long-range, American 175 mm M107 Self-Propelled Gun. The United States had air superiority though many aircraft were lost to surface-to-air missiles and anti-aircraft artillery. U.S. airpower was credited with breaking the siege of Khe Sanh and blunting the 1972 Easter Offensive against South Vietnam. At sea, the U.S. Navy had the run of the coastline, using aircraft carriers as platforms for offshore strikes and other naval vessels for offshore artillery support. Offshore naval fire played a pivotal role in the Battle of Huế in February 1968, providing accurate fire in support of the U.S. counter-offensive to retake the city. The Vietnam War was the first conflict that saw wide scale tactical deployment of helicopters. The Bell UH-1 Iroquois nicknamed "Huey" was used extensively in counter-guerilla operations both as a troop carrier and a gunship. In the latter role it was outfitted with a variety of armaments including M60 machine guns, multi-barreled 7.62 mm Miniguns and unguided air-to-surface rockets. The Hueys were also successfully used in MEDEVAC and search and rescue roles. Two aircraft which were prominent in the war were the AC-130 "Spectre" Gunship and the UH-1 "Huey" gunship. The AC-130 was a heavily armed ground-attack aircraft variant of the C-130 Hercules transport plane; it was used to provide close air support, air interdiction and force protection. The AC-130H "Spectre" was armed with two 20 mm M61 Vulcan cannons, one Bofors 40mm autocannon, and one 105 mm M102 howitzer. The Huey is a military helicopter powered by a single, turboshaft engine, and approximately 7,000 UH-1 aircraft saw service in Vietnam. At their disposal ground forces had access to B-52 and F-4 Phantom II and others to launch napalm, white phosphorus, tear gas and chemical weapons as well. The aircraft ordnance used during the war included precision-guided munition, cluster bombs, a thickening/gelling agent generally mixed with petroleum or a similar fuel for use in an incendiary device, initially against buildings and later primarily as an anti-personnel weapon that sticks to skin and can burn down to the bone. The Claymore M18A1, an anti-personnel mine was widely used, and is command-detonated and directional shooting 700 steel pellets in the kill zone.