WASHINGTON — U.S. and British ships and submarines launched the first phase of a missile assault on Libyan air defenses Saturday and a senior American defense official said it was believed substantial damage was inflicted.

In the strikes, 112 Tomahawk cruise missiles were fired at more than 20 coastal targets to clear the way for air patrols to ground Libya’s air force.

While U.S. defense officials cautioned that it was too early to fully gauge the impact of the onslaught, the official said that given the precision targeting of the Navy’s cruise missiles, they felt that Libya’s air defenses suffered a good deal of damage.

Explosions continued to rock the coastal cities, including Tripoli. Navy Vice Adm. Wiliam E. Gortney, director of the Pentagon’s Joint Staff, would not discuss future operations But defense officials said military action was likely to continue.

The official spoke on grounds of anonymity because the ongoing mission.

In announcing the mission during a visit to Brazil, President Barack Obama said he was reluctant to resort to force but was convinced it was necessary to save the lives of civilians. He reiterated that he would not send American ground troops to Libya.

“We cannot stand idly by when a tyrant tells his people there will be no mercy,” he said in Brasilia.

While U.S. defense officials said it was too early to gauge the impact of the onslaught, one senior official said that given the precision targeting of the Navy’s cruise missiles, they believe Libya’s air defenses suffered a good deal of damage.

It was clear the U.S. intended to limit its role in the Libya intervention, focusing first on disabling or otherwise silencing Libyan air defenses, and then leaving it to European and perhaps Arab countries to enforce a no-fly zone over the North African nation.

Gortney told reporters the cruise missile assault was the “leading edge” of a coalition campaign dubbed Operation Odyssey Dawn. Its aim: prevent Moammar Gadhafi’s forces from inflicting more violence on civilians -- particularly in and around the rebel stronghold of Benghazi -- and degrading the Libyan military’s ability to contest a no-fly zone.

“This is not an outcome the U.S. or any of our partners sought,” Obama said from Brazil, where he is starting a five-day visit to Latin America. “Our consensus was strong, and our resolve is clear. The people of Libya must be protected, and in the absence of an immediate end to the violence against civilians our coalition is prepared to act, and to act with urgency.”

A chief target of Saturday’s cruise missile attack was Libya’s SA-5 surface-to-air missiles, which are considered a moderate threat to some allied aircraft. 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.

Gortney initially had said that it could take as long as 12 hours to assess the effectiveness of Saturday’s strikes. Then a high-altitude Global Hawk unmanned surveillance plane would overfly the target areas to get a more precise view, the admiral said. He would not say how long the attacks on Libyan air defenses would last, but he stressed that Saturday’s assault with cruise missiles was the first phase of a multi-stage mission.

Senate Majority Leader Harry Reid, in a statement late Saturday, said, “I support the actions taken today by our allies, with the support of several Arab countries, to prevent the tyrant Moammar Qaddafi from perpetrating further atrocities on the people of Libya.”

“And I support the president’s decision to deploy U.S. assets to help those allies to enforce a no-fly zone to protect Libyan civilians as laid out in the United Nations resolution,” the Nevada Democrat said. “This U.S. military action was not taken lightly, and it was done in concert with a broad international coalition.”

Defense Secretary Robert Gates, who was scheduled to fly to Russia on Saturday afternoon to begin a week-long overseas trip, postponed his departure for 24 hours. Pentagon press secretary Geoff Morrell said Gates decided he should remain in Washington to monitor developments in Libya at the outset of U.S. strikes.

Gates had been 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.

At a news conference in Paris, Clinton said Gadhafi had left the world no choice but to intervene urgently and forcefully to protect further loss of civilian life.

“We have every reason to fear that, left unchecked, Gadhafi would commit unspeakable atrocities,” she told reporters.

Clinton said there was no evidence that Gadhafi’s forces were respecting an alleged cease-fire they proclaimed and the time for action was now.

“Our assessment is that the aggressive action by Gadhafi’s forces continues in many parts of the country,” she said. “We have seen no real effort on the part of the Gadhafi forces to abide by a cease-fire.”

In addition to the three submarines and two destroyers, the U.S. Navy ships in the Mediterranean include two amphibious warships, the USS Kearsarge and USS Ponce, and a command-and-control ship, the USS Mount Whitney.