Hours later, two American military police officers were injured and one went into shock after two attacks with rocket-propelled grenades on a northwest Baghdad police station, said Lt. Clint Mundinger of the US Army’s 709th Military Police Battalion. He had no further details.

The attacks were the latest in what have been three lethal days for American troops in postwar Iraq — seven soldiers dead in attacks and accidents since Sunday and well over a dozen injured.

In Fallujah, six Iraqis were captured and were being interrogated yesterday afternoon, said Maj. Randy Martin, a spokesman for the US Army’s V Corps. The attackers used rocket-propelled grenades and small arms in the attack, a statement from US Central Command said, but Martin said the grenade was thrown by hand.

All the US soldiers hit were from the 3rd Armored Cavalry Regiment, based in Fort Carson, Colorado, Martin said.

“Who knows what they were thinking when they engaged US soldiers?” Martin said. “I know we suffered casualties, and the enemy paid a price for those casualties.”

US occupying forces have run into trouble before in Fallujah, some 50 km (30 miles) west of Baghdad. Its 200,000 people benefited greatly from Saddam Hussein’s Baath regime. Saddam built chemical and munitions factories that employed Fallujah’s young men and gave others jobs in his elite Republican Guard.

“Fallujah has been an area of concern for us,” Martin said. But he said conditions had been improving before yesterday’s attack.

Protests against the army’s presence in Fallujah turned violent when US soldiers fired on crowds on April 28 and April 30, killing 18 Iraqis and wounding at least 78. The soldiers said then that they were defending themselves and the crowd fired first, but Iraqis said no shots were fired at the Americans.

Details of the incident, which occurred just after midnight yesterday at a traffic checkpoint in Fallujah, were hazy, and accounts varied.

Initial reports said the Americans were fired upon from many directions, including from a mosque, US military officials said. But townspeople said only the two dead men had opened fire, and were quickly cut down by US forces.

Bashir Jasim, who lives in the area, said the two Iraqis stopped their pickup truck at a traffic checkpoint, stepped out and opened fire. They were killed immediately by the Americans, he said. Other townspeople recounted very similar versions of the events.

Martin said two vehicles had pulled up the checkpoint together, and a search of the first turned up some weapons. Just as the guns were discovered, men in the second vehicle opened fire and threw a grenade.

The intensity of the assault “would suggest the possibility” that it had been coordinated beforehand by the vehicles’ occupants, said Capt. Tom Bryant, another V Corps spokesman. In New York, US Secretary of Defense Donald Rumsfeld said that Iraq faced a rough transition to democracy, and warned that any effort to remake the country in Iran’s image would be “aggressively put down.”

In a speech to the Council on Foreign Relations, Rumsfeld stressed that the United States had no intention of imposing an “American template” on the Iraqi people, but pledged that US troops would remain in Iraq as long as they were needed.

“The transition to democracy will take time. It will not be a smooth road,” Rumsfeld said, adding that “trial and error” would be an inevitable part of the process.

“The efforts will not be perfect. Course corrections will be needed,” he said, calling on all parties involved to exercise patience and restraint. “If the Iraqi people take hold of their country...and claim their place as responsible members of the international community, then the world could well have a new model for a successful transition from tyranny to self-reliance,” he said.

While welcoming regional offers of cooperation, Rumsfeld said that “interference” in Iraq by its neighbors or their proxies “will not be permitted.”

Rumsfeld particularly warned Iran against seeking to mould the future path of Iraq’s social and political development. “Indeed, Iran should be on notice: Efforts to try to remake Iraq in Iran’s image will be aggressively put down,” he said.

The remark came amid US charges that Tehran is seeking to influence events in Iraq, harboring senior Al-Qaeda leaders and developing nuclear weapons.