Navy Vice Adm. William E. Gortney, director of the Pentagon's Joint Staff, told reporters the cruise missile assault was the "leading edge" of a coalition campaign dubbed Operation Odyssey Dawn.
He said it would take six to 12 hours to assess the damage, and if the main targets — Libya's SA-5 surface-to-air missiles — were taken out, then it would be safe to send an unmanned Global Hawk surveillance drone to get a better picture of the area.
Libya's overall air defenses are based on older Soviet technology but Gortney called them capable and a potential threat to allied aircraft.
Also targeted: early warning radars and unspecified communications facilities, Gortney said. The U.S. military has extensive recent experience in such combat missions; U.S. Air Force and Navy aircraft repeatedly attacked Iraq's air defenses during the 1990s while enforcing a no-fly zone over Iraq's Kurdish north.
Cruise missiles are the weapon of first choice in such campaigns; they do not put pilots at risk, and they use navigational technologies that provide good precision.
The first Tomahawk cruise missiles struck at 3 p.m. EDT, Gortney said, after a one-hour flight from the U.S. and British vessels on station in the Mediterranean.
They were fired from five U.S. ships — the guided-missile destroyers USS Stout and USS Barry, and three submarines, USS Providence, USS Scranton and USS Florida.
The U.S. has at least 11 naval vessels in the Mediterranean, including three submarines, two destroyers, two amphibious warfare ships and the USS Mount Whitney, a command-and-control vessel that is the flagship of the Navy's 6th Fleet. Also in the area are Navy P-3 and EP-3 surveillance aircraft, officials said.
Gates was skeptical of getting involved in Libya's civil war, telling Congress earlier this month that taking out Libya's air defenses was tantamount to war. Others have worried that the mission could put the U.S. on a slippery slope to deeper involvement in yet another Muslim country — on top of the wars in Iraq and Afghanistan.
Hours after Secretary of State Hillary Rodham Clinton attended an international conference in Paris that endorsed military action against Gadhafi, the U.S. and Britain kicked off their attacks. Clinton said Gadhafi had left the world no choice but to intervene urgently and forcefully to protect further loss of civilian life.
Associated Press writer Robert Burns contributed to this report.