Air raid sirens blared and explosions rocked Baghdad tonight, signaling a second wave of U.S. and British attacks. Anti-aircraft artillery lighted the night sky over the Iraqi capital.

After at least a dozen explosions, a pall of white smoke hung over the center of the Iraqi capital, and the skyline above parts of the city glowed orange as if buildings were on fire.

After the first explosion was heard, anti-aircraft fire responded and tracer bullets crisscrossed in the sky.

It was not immediately clear what had been hit by the incoming blasts, but U.S. military officials had said the second night of airstrikes would be bigger and more devastating than the first.

In Washington, officials at the Pentagon, who spoke on condition of anonymity, said cruise missiles had been fired deep into Iraq in the second attack, and Navy strike aircraft had fired laser-guided bombs along Iraq's borders.

At the government's press office in downtown Baghdad, witnesses reported one missile flying overhead Thursday night. After the first explosion at 1:57 p.m. EST, anti-aircraft fire was unleashed and tracer bullets darted into the air.

The blasts grew increasingly loud as they came nearer the downtown. One missile appeared to have hit the vicinity of the government's Military Industrial Corporation, from where the smoke drifted into the sky.

There was no immediate word on casualties from the Thursday night attack, and foreign reporters were not allowed to move freely around the capital.

President Clinton, at a press conference in Washington today, rejected suggestions that he launched military strikes at Iraq to divert attention from the pending impeachment vote in Congress.

"This was the right thing for the country," he said at a morning White House briefing. "We gave Saddam Hussein chance after chance. ... We are going to complete this mission."

In Congress, the House today approved a resolution supporting military troops in the Gulf, despite skepticism about the timing of the attack.

Chief U.N. arms inspector Richard Butler defended his report that formed the grounds for U.S.-British air strikes against Iraq, saying it "danced to no one's tune."

"That report is honest, factual and objective and sadly the conclusion is correct: that Iraq's promise of full cooperation was not kept," he said.

In his first public comments since the attacks, he said his report "was not written for anyone's purpose."

U.S. officials said today that the first wave of attacks � which targeted suspected weapons sites, airfields and other command and control infrastructure � had been a success, leveling special forces, intelligence posts in Baghdad. U.S. forces attacked more than 50 targets in the first wave, officials said.

"Our forces are performing well," Secretary of Defense William Cohen said in an afternoon news conference. Cohen said that no American casualties had been reported in the first wave.

The initial attacks, intended to degrade Iraq's air defenses, in theory will reduce the risk for U.S. and British combat aircraft flying over Iraq. With more than 200 cruise missiles already fired and up to 300 more on the way, the Pentagon nevertheless says it will be impossible to completely destroy Iraq's air defense system.

Gen. Henry H. Shelton, chairman of the Joint Chiefs of Staff, said sea-launched Tomahawk cruise missiles slammed into key Iraqi command structures in the opening wave of what was expected to be a four-day air campaign.

Iraqi Foreign Minister Mohammed al-Sahhaf, in the first government news conference since the airstrikes began, said cruise missiles had inflicted ''heavy casualties'' in populated areas of Baghdad in the first wave of attacks but he declined to give a specific figure.

The laser-guided bombs being dropped by Navy F-18s and Air Force F-16s are among the most accurate weapons in the U.S. inventory and are being used against targets such as command bunkers, buildings and weapons storage sites where accuracy is at a premium.

Sketchy spy satellite and U-2 spy plane photographs indicate that the first wave of strikes hit home, but defense officials say that even if the strikes are successful, the U.S. military will have to remain on patrol in the Persian Gulf indefinitely.

Baghdad shook with explosions and volleys of anti-aircraft flak cascaded like fireworks through the darkened skies early in the morning during the first round of airstrikes launched by U.S. and British forces. At least two people were killed and more than 30 injured from the initial strikes, a doctor said.

Cohen told AP Radio that initial assessments indicate "substantial damage was done to a number of targets.'' But he added, "I'm not trying to classify them as being totally successful at this point.''

Cohen responded curtly to Iraqi claims that civilian targets were struck by stray cruise missiles: "We've got the right targets.''

About 2,000 Army troops and 35 aircraft were en route to the Persian Gulf region today, Cohen said. The troops are bound for Kuwait where they will join 3,000 U.S. soldiers already on hand in the unlikely event Iraq strikes out in response to the air assault.

The air strikes, dubbed "Operation Desert Fox,'' are expected to be over by this weekend, when the Muslim holy month of Ramadan begins. But Cohen said the U.S. military presence in the Persian Gulf will remain as long as Iraq remains a threat.

By Friday, with the arrival of the USS Carl Vinson carrier battle group, the Navy will be able to launch upward of 500 more cruise missiles at Iraq, defense officials said.

After the first cruise missiles were launched, Navy EA-6B Prowler fighters flying off the carrier USS Enterprise in the Persian Gulf hit Iraqi air defense radars with HARM missiles.

Once those air defenses are down, some of the 246 combat planes now in the Gulf or soon to arrive will launch follow-up strikes. Four Air Force B-1s, 15 B-52s, the latter capable of launching cruise missiles, dozens of bomb-dropping Navy F-14s and Air Force F-16s, 10 radar-evading F-117 stealth fighter-bombers, and an array of British combat aircraft are among the air armada.

The heavy reliance on cruise missiles and radar-evading aircraft point to a key Pentagon goal � so far successful � of avoiding casualties. Later phases of the air campaign will involve conventional U.S. warplanes flying more dangerous bombing missions over Iraqi territory.

U.S. military officials began planning these strikes in mid-November, when Iraq narrowly avoided strikes by making last-minute concessions to weapons inspectors.