Attacks on American troops in Iraq have declined in recent weeks, but strikes against Iraqis are up, officials say.
Vehicles packed with explosives struck police stations on Saturday
At the same time, US forces have stepped up attacks on suspected insurgents, US administrator Paul Bremer and General John Abizaid said.
"The security situation has changed," Mr Bremer said, adding that this had come about because the US was now taking the fight to the enemy.
Mr Bremer said the attackers would not stop Iraqis from working with the US.
"They have failed to intimidate the coalition. They have now begun a pattern of trying to intimidate innocent Iraqis," he said.
"They will not succeed. If Saddam [Hussein] taught the Iraqis nothing else, it was how to endure the depredations of thugs," he said.
A grenade at a hotel used by US contractors in the northern city of Kirkuk injured three Iraqis as Mr Bremer and General Abizaid were speaking.
US soldiers shot dead three Iraqis who they said were planting a roadside bomb near the town of Falluja on Monday night.
Two other Iraqis fled the scene, one with the explosive device, Central Command said in a statement.
At least 250 Iraqis are believed to have been killed in anti-occupation resistance attacks since 1 May. No official figures are kept of the Iraqi death toll.
At the news conference, General Abizaid said "regime loyalists", not foreign fighters, were the main opponent of US-led forces.
He also reiterated that the US did not intend to keep its forces in Iraq indefinitely.
"There is no doubt that American forces will leave Iraq... when we are no longer needed," he said.
But the BBC's Peter Greste in Iraq says the formulation leaves a lot of room for interpretation, and that the US would probably have troops in Iraq for years.
The Iraqi Governing Council announced on Monday a timetable for return to self-rule.
The letter from current council President Jalal Talabani to the United Nations anticipates a new transitional government being elected by the end of June 2004.
It says dates have been agreed with the US and UK, and calls for a new UN security council resolution on Iraq.
At least 18 Iraqis were killed when two police stations north of Baghdad were bombed within minutes of each other on Saturday.
Six Iraqi policemen and three civilians were killed when a car packed with explosives rammed into the police station at Khan Bani Saad, 50 kilometres (30 miles) north-east of Baghdad.
About 15 kilometres north, a second car bomb rocked the main police station at Baquba, killing seven policemen and two civilians.
The towns lie in the "Sunni triangle" area - a stronghold of supporters of Saddam Hussein and scene frequent attacks against US forces by insurgents.