Baghdad, Iraq ? A roadside bomb exploded north of Baghdad on Wednesday, killing three U.S. soldiers in the deadliest attack on Americans since Saddam Hussein’s capture. This morning, a rebel rocket shattered windows on a Baghdad hotel filled with Western contractors and journalists.

The attack on the Ishtar Sheraton Hotel around dawn today came just hours after insurgents struck the same hotel with a mortar shell. There were no reports of injury in either attack.

At the same time today, another explosion and gunfire resounded in the city, setting off sirens that wailed for several minutes. U.S. soldiers trained their guns on the street outside the hotel. But military officials said they had no immediate details on the violence.

“We just heard a loud boom and everybody woke up,” said a hotel desk clerk, who wasn’t further identified.

In the Wednesday night blast, the 60mm direct-lay mortar shell hit a barrier, or outer wall, on the heavily barricaded 19-story hotel, which rises from the east bank of the Tigris River, said Capt. Jason Beck of the U.S. Army’s 1st Armored Division.

Beck told The Associated Press that Iraqi security guarding the hotel immediately fired at the guerrillas, who fled. Late Wednesday and early today, distant explosions were heard in central Baghdad as the U.S. military bombarded suspected rebel positions.

The hotel attacks came after a string of separate bombings that killed six civilians and a suicide bomber in addition to the three American soldiers.

Until Wednesday, U.S. military commanders had said the number of daily rebel attacks were slowing in recent weeks — even as they braced for increased violence around the Christmas holiday.

Wednesday’s fighting began before dawn, when the 1st Armored Division unleashed an artillery barrage on three rebel targets in southwest Baghdad, aided by Air Force jet fighters and gunships.

Elsewhere, U.S. troops continued their stepped-up raids on homes in several towns that led to the arrest of a Sunni sheik said to be close to the most wanted man in Iraq. Troops rounded up dozens of guerrilla suspects in strongholds of U.S. resistance, saying they were capitalizing on intelligence from interrogations and documents seized in the Dec. 13 capture of Saddam, the former dictator.

At 9 a.m. Wednesday, three U.S. soldiers were killed by a roadside bomb that hit a military convoy near Samarra, a town north of Baghdad where insurgents have often launched attacks.

In northern Iraq, a suicide bomber detonated an explosives-packed car in front of the Kurdish Interior Ministry in the city of Irbil, near Kirkuk, U.S. Army Brig. Gen. Mark Kimmitt said in Baghdad.

Four civilians were killed — two guards, a 13-year-old girl and a passing taxi driver — along with the bomber, said Interior Minister Karim Sinjar. He said 101 people were injured in the 11 a.m. explosion, two seriously.

Also Wednesday, two people were killed and two injured — all minibus passengers — when a roadside bomb detonated in a Baghdad traffic tunnel, hospital officials said. The bomb exploded in the traffic-packed Shurta tunnel around noon.

Improvised roadside bombs have become the favored rebel tactic in recent months. The cleverly concealed bombs have killed numerous American troops.

Kimmitt said the overnight U.S. bombardment, which residents heard until about 2 a.m., was the launch of Operation Iron Grip, targeting an area that had been used to fire mortars at U.S. troops. The bombardment consisted of “targeted attacks, very precise,” Kimmitt said.

He also indicated that the operation was a show of force in the military’s latest offensive against Baghdad’s guerrillas.