BAGHDAD, Iraq (AP) -- Anti-aircraft guns started firing in Baghdad for the fourth straight night, the start of a new wave of punishing U.S. and British airstrikes.

Gun batteries on rooftops sent tracer shells streaking across the sky after dusk. The guns began firing minutes after sirens wailed across the capital.

In Washington, a senior White House official said today that a fourth round of airstrikes had begun.

There were unconfirmed reports of muffled explosions on the outskirts of Baghdad.

The United States and Britain launched a series of airstrikes overnight Wednesday to punish Iraq for its alleged obstruction of U.N. weapons inspectors seeking evidence of the country's weapons of mass destruction.

In the latest attacks at dusk today, the gun batteries, positioned in the city's eastern outskirts, fell silent in Baghdad after four minutes. But about 15 minutes later they started firing again.

Reporters on the roof of the Information Ministry building in the city center saw no sign of incoming missiles nor did they hear the explosions.

Shortly before the anti-aircraft guns began to fire again in Baghdad, President Clinton warned today that the United States was to strike again.

``We stand ready to use force again if Saddam takes threatening action, such as seeking to reconstitute his weapons of mass destruction, menacing his neighbors or his own Kurdish citizens or challenging allied aircraft,'' he said.

During another attack before dawn today, reporters in the Iraqi capital saw fiery streaks from what appeared to be Tomahawk cruise missiles arcing across the sky and landing in giant fireballs. Warheads exploded a few seconds later, lighting the pre-dawn horizon in a red glow that silhouetted palm trees against the dark sky.

Iraqi anti-aircraft gunners on rooftops opened fire, filling the sky with booms and bright red flashes. Orange tracer bullets streaked high above.

When the attack ended, ambulances or fire trucks with sirens blaring were heard speeding away. Officials, who must escort journalists wishing to tour the city, refused to take them to the targeted areas.

Baghdad residents said damage from the earlier attacks today and Friday was heavy and that a number of buildings in the center of the capital -- including palaces, security headquarters and offices of the ruling Baath Party -- had been hit.

Western reporters staying at the Al-Rashid Hotel in central Baghdad saw explosions in nearby areas known to contain palaces, a headquarters of the Special Security service and the command center for the Popular Army, which is a paramilitary force.

The walls of the Military Industrialization Corp., which oversees state-run factories, were still standing, but the building appeared to have been gutted by missiles.

Today's first attack appeared to be the toughest yet against Baghdad.

Before today, the United States fired nearly 300 Tomahawk cruise missiles from ships in the Gulf or from B-52 bombers. The missiles carry warheads weighing up to 3,000 pounds.

Iraq says 25 people were killed and 75 injured in the first two nights of attacks.

The state-run Iraqi News Agency said today that five more people were killed and 21 wounded in a missile attack on the Al-Riyad neighborhood of the northern town of Kirkuk.

The report said an agricultural training center and a number of houses were damaged.

INA also referred later to a march held Saturday for 68 people killed in the U.S. and British attacks, but Mohsen al-Tarfa, deputy director of the government press center, said the 68 dead were not killed in the attacks but were victims of U.N. sanctions.

Iraq has complained that thousands have died because limited medicines have entered the country under the sanctions.

Meanwhile, a second U.S. aircraft carrier battle group -- led by the USS Carl Vinson -- moved into the Gulf to join the USS Enterprise group, U.S. Navy officials said.

The pre-dawn explosions were not preceded by air raid sirens, implying that for the first time the strikes eluded Iraqi air defenses.

When the reverberations from the pre-dawn explosions subsided about 4:45 a.m., the winter air filled with the sounds of muezzins calling from mosque loudspeakers, informing Muslims to eat their last meal before beginning the first day of fasting for the holy month of Ramadan.

On Friday, in an address to the nation broadcast on television, President Saddam Hussein vowed that Iraq ``will not compromise or kneel in the face of injustice.''

State-run newspapers today echoed Saddam's theme. A banner headline in the Al-Jumhuriya daily read: ``We shall resist and fight in the name of God.''

Officials at the Oil Ministry said today that crude exports under the U.N. approved oil-for-food program were continuing despite the military attacks.

The officials said an average of 1.8 million barrels per day were leaving through Mina al-Bakr port and terminals at the Turkish port of Ceyhan on the Mediterranean Sea. They spoke on customary condition of anonymity.

During a Pentagon briefing on Friday, the Joint Chiefs of Staff chairman Gen. Henry Shelton said that U.S. forces have hit more than 75 targets in the last two nights.

But initial bomb-damage assessments indicate that only a small number of the targets were destroyed or severely damaged even though Iraq offered virtually no resistance.

For example, of 27 Iraqi surface-to-air missile facilities attacked, only one was destroyed and eight suffered no damage.

The Iraqi military claimed its anti-aircraft batteries had shot down 77 incoming missiles so far.

The United States says the aim of Operation Desert Fox is to hurt Iraq's ability to make weapons of mass destruction, threaten its neighbors and attack U.S. interests in the region.

But Iraq's Deputy Prime Minister Tariq Aziz questioned those goals.

``The real objective of this aggression is to show that the United States is the sole superpower,'' he told reporters Friday.

He said the strikes targeted the old Defense Ministry building, which houses only administrative offices, a refinery in the southern city of Basra, a Trade Ministry warehouse and a radio-television transmission facility.

The Pharmacy College of Baghdad and the Museum of Natural History were also hit.

Aziz did not say which military targets had been hit.

But in Washington, U.S. officials showed aircraft videotape of laser-guided bombs slamming into what they said were military targets, including radar stations, a missile storage facility and a missile production plant.

Aziz said almost all potential weapons sites that U.N. weapons inspectors had been monitoring have been hit.

The inspectors, working in the country since 1991, withdrew on Wednesday after U.N. weapons chief Richard Butler complained of a lack of cooperation from Iraq. Butler's critical report triggered the U.S.-British attack.

Until the monitors certify that Iraq is free of biological and chemical weapons and long-range missiles, the U.N. Security Council will not lift economic sanctions imposed on Iraq for its 1990 invasion of Kuwait.