Top U.S. civilian and military officials also visited the base Saturday amid speculation in Washington that a decision to move against the insurgents might come as soon as this weekend. White House officials declined Saturday to comment on reports that President Bush was discussing whether to attack with top advisers and military officials at Camp Fallujah declined to say why U.S. Iraq administrator L. Paul Bremer and Gen. John Abizaid, the top military commander in the region, had visited the base.

Meanwhile, violence flared throughout the country, including rare suicide boat attacks on Iraqi oil facilities in the Persian Gulf.

Four Americans were killed in a dawn rocket attack on their base 12 miles north of Baghdad. Two other Americans died later as their eight-member boarding party tried to intercept a boat near an Iraqi oil installation. The boat exploded as the group approached it, according to news reports.

Two other boats were sighted later approaching oil tankers. They also exploded as security forces attempted to intercept them. No oil facilities were damaged in the attacks.

Elsewhere, Marines near Fallujah called in an AC-130 combat aircraft to strafe a suspected insurgent position, killing 30 suspected enemy; a roadside bomb killed two Iraqi police officers and two civilians in Tikrit, and at least six Iraqis were killed and 38 wounded when insurgent mortar rounds fell on a crowded market in the sprawling Baghdad neighborhood of Sadr City.

Coalition military spokesman Brig. Gen. Mark Kimmitt blamed the mortar attack on Iraqi insurgents and said the target was an old cigarette factory that had been used as a military base by the Americans, but that the shells missed and landed in the marketplace. Some residents blamed U.S. helicopters for the assault, an allegation Kimmitt denied.

The casualties came as U.S. commanders debated what to do about the standoff at Fallujah, where insurgents remain in control of most of the city, while a gathering U.S. force controls most of the avenues in and out of the city.

Hundreds of Marines arrived overnight at Camp Fallujah, a former Iraqi military base that has become the headquarters for U.S. operations in Anbar province, where U.S. and insurgent Iraqi forces this month have fought some of the bloodiest battles since U.S. troops entered Iraq 13 months ago.

In Baghdad, Kimmitt made it clear that U.S. patience is wearing thin with the standoff.

"We will try to resolve this peacefully, but our patience is not limitless," he told reporters. "Should there not be a good faith effort demonstrated by belligerents inside Fallujah, the coalition is prepared to act."

But Kimmitt seemed to be aware that any decision to attack Fallujah would be broadly unpopular in Iraq and would risk alienating many Iraqis who have favored U.S. involvement in their country. During earlier fighting in the city, thousands of Iraqis collected food and medical supplies for residents of Fallujah amid accusations that U.S. forces had caused hundreds of civilian deaths.

Kimmitt attempted to build the military's case for moving into Fallujah, unveiling a "report card" demonstrating how insurgents in Fallujah had failed to implement any of the conditions of a two-week-old ceasefire U.S. officials have said insurgents have violated repeatedly.

Kimmitt said that in the previous 24 hours alone, insurgents had violated the ceasefire 15 times.

"What is strongly represented as a gallant, noble resistance operation inside Fallujah is nothing more than foreign fighters and former (Saddam loyalists), who are holding that city hostage," Kimmitt said. "But what they are holding that city hostage from is the benefits of Iraqi sovereignty."

Kimmitt said a number of softer measures Marines had implemented, including a later curfew, allowing Fallujan refugees to return home and firing only when fired upon, have not been reciprocated.

He said insurgents have not disarmed or turned over heavy weapons. Nor have city leaders condemned the insurgents or launched an investigation into the deaths of four American contractors whose bodies were mutilated there late last month.

"The coalition certainly understands it can use force at any time. It has more than sufficient force to go in, conduct offensive operations and end the hostage taking of the town of Fallujah," Kimmitt said.

But such a battle is certain to bring heavy casualties to both sides. U.S. forces in Anbar province have reported that the insurgents appear to be well trained militarily and have taken their most serious casualties in 13 months of combat as they try to dislodge the rebels.

While tens of thousands of civilians have fled the city, perhaps as many as 200,000 remain inside the city.

Renewed fighting at Fallujah might also spark fighting in other parts of Iraq, challenging U.S. and coalition forces already spread thin by weeks of challenges from insurgent forces and increasing Iraqi anger at the U.S.-led occupation.

Officials are wary of moving into the holy Shiite city of Najaf in central Iraq, where rebel cleric Muqtada al Sadr has vowed to fight a U.S. pledge to kill or capture him. Coalition forces have retaken most of the cities in the south that Sadr's Mahdi Army militia captured earlier this month, but Sadr has vowed he will not disarm.

Fighting was reported Saturday in Karbala.

The failed attacks at the oil terminals in the Persian Gulf were a new wrinkle in Iraq's cycle of violence and suspicion immediately fell on al-Qaida terrorists, who are also suspected in car bomb attacks early last week than claimed 70 lives in the city of Basra.

The suicide-boat attacks recalled the 1997 al-Qaida attack on the USS Cole, though there was no claim of responsibility.

The oil terminal where the attacks took place exports nearly 1.9 million barrels of oil per day, a major source of Iraq's funding.

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(Knight Ridder correspondents Bob Moran and Carol Rosenberg contributed to this report.)