WASHINGTON - A second round of U.S.-led air attacks was launched Thursday night in Baghdad, but U.S. military officials said the assault was not the beginning of the massive air campaign the Pentagon has planned.

The air attacks underway in Baghdad were sea-launched Tomahawk cruise missiles fired at Special Republican Guard strongholds in Baghdad, according to two senior defense officials with direct knowledge of the operation, who asked not to be identified.

Thursday's airstrikes were not the beginning of the massive air assault that the Pentagon plans to unleash later, three officials said. Two of the officials said Thursday's strikes involved a smaller number of Tomahawks than Wednesday's opening volley, which numbered approximately 40.

The latest attack included Tomahawk cruise missiles fired by British ships as well as American ships, one official said.

A huge plume of smoke could be seen from the west bank of the Tigris in central Baghdad but it could not be determined what was hit.

Meanwhile, F-14 and F-18 jets took off from the USS Theodore Roosevelt in the eastern Mediterranean, armed with missiles and bombs.

The new attacks came shortly after senior U.S. military leaders said the war in Iraq may not be over quickly, and "there will be casualties."

The war's opening salvos before dawn Thursday were aimed directly at Iraq's leaders, including President Saddam Hussein .

Commanders relied on more than 40 cruise missiles launched from Navy ships and submarines in the Persian Gulf and the Red Sea, and 2,000-pound precision-guided bombs dropped by Air Force stealth fighter jets, military officials said.

Iraq may have set fire to three or four oil wells in south Iraq, Defense Secretary Donald Rumsfeld said.

The Pentagon has overhead photos of the fires, which were located less than two miles north of the Kuwaiti border and about 50 miles southwest of the city of Basra, a defense official said later on condition of anonymity.

Point Chiefs of Staff Chairman Gen. Richard Myers warned that the war won't be easy for the United States.

"We do not regard combat as an easy task," Myers said at the Pentagon. "Warfare is dangerous. We will have casualties."

The Pentagon assessed the damage Thursday from its initial strikes against targets in Iraq and primed for a broader assault involving 250,000 U.S. and coalition forces.

"We have a serious task before us, to remove that regime," Rumsfeld told reporters at the Pentagon. "This is a process that takes some time."

A senior military official, speaking on condition of anonymity, said military intelligence was seeing some evidence of disarray in Iraqi leadership. It was too early to say if any of them were killed or wounded.

Rumsfeld said the assault "was the first. It will not likely be the last."

Rumsfeld was asked whether military planners knew Saddam's location Wednesday night.

"We had what I would characterize as very good intelligence that it was a senior Iraqi leadership compound. We do not know what the battle damage assessment" will be, he said.

Officials said the surprise attack was the product of a complex operation that benefited from electronic spying and other intelligence, special military operations, and changes in technology that permitted military chiefs to more quickly reconfigure the cruise missiles for a special, pinpointed attack.

Rumsfeld warned Iraqis not to go to work, but to stay in their homes and listen to coalition radio broadcasts.

"The day of your liberation may soon be at hand," Rumsfeld said. "The days of the Saddam Hussein regime are numbered. We continue to feel there is no need for a broader conflict if the Iraqi leaders act to save themselves and to prevent such further conflict."

A helicopter carrying U.S. special forces crashed inside southern Iraq hours before the missile strikes Wednesday night, a senior defense official said. There were no casualties and the troops on board were all taken out safely, said the official, who spoke on condition of anonymity. The official said the military was taking steps to destroy the helicopter rather than let it fall into Iraqi hands.

The incident makes clear the Pentagon was using a well-worn war tactic of dropping special commando forces behind enemy lines before the opening of the rest of the campaign.

Officials declined to say exactly where the crash occurred. But a widely discussed part of the war plan has been to send special forces into the country to secure oil wells, suspected chemical weapons sites and other strategic locations — as well as to search out Iraqi leadership.

The defense official also said a small plane headed from Iraq toward a Marine expeditionary force position in Kuwait but crashed short of its mark. The Marines donned gas masks because of fears that the plane could have been carrying chemical weapons, the official said. No agents were detected