Chief administrator L. Paul Bremer III said the insurgents' recent attacks on the coalition itself were not having the desired effect, so they were turning to the Iraqis who help occupation forces.
"The security situation has changed," Bremer said at a press conference.
"They have failed to intimidate the coalition," he said. "They have now begun a pattern of trying to intimidate innocent Iraqis. They will not succeed."
Large explosions were heard after sundown Tuesday in central Baghdad but the precise location was unclear. A U.S. military spokesman said only that he had heard the explosion but "we don't know anything yet."
Insurgents have fired on the U.S. headquarters compound, known as the "green zone," this month but there has been no firing in that area since the U.S. military launched "Operation Iron Hammer" to strike at guerrilla hideouts in the city.
Following the blast, which occurred after 8 p.m., sirens could be heard briefly from the west bank of the Tigris River, where the green zone is located. Sporadic small arms fire could also be heard from the same area.
In other developments:
The U.S. military staged more raids as part of its more aggressive campaign against Iraqi insurgents. The Fourth Infantry Division says its soldiers arrested 18 Iraqis during nearly 200 raids over the past 24 hours in a sector north of Baghdad. Troops also seized assault rifles, grenades, blasting caps and other munitions during the raids.
Residents of Husaybah, located about six miles from Syrian border, said U.S. troops sealed off the town four days ago and have been searching houses for weapons and insurgents and detaining dozens of people. U.S. officials have long complained about movement of fighters and weapons from Syria into this country.
Lt. Gen. John Abizaid, head of U.S. Central Command, said that the number of daily attacks on coalition forces were down by about half over the last two weeks. He gave no figures but U.S. officials have said U.S. forces were being attacked on average of 30-35 times a day.
"In the past two weeks, these attacks have gone down, attacks against coalition forces, but unfortunately we find that attacks against Iraqis have increased," Abizaid said. He said the attacks had increased not only in number but in severity.
The series of attacks includes two car bombs last weekend at police stations in Baqouba and Khan Bani Saad, the assassination Saturday of a police colonel in Mosul and the killing Sunday of a police chief in Latifiyah near Baghdad.
Abizaid said there were some foreigners fighting with the insurgents but their numbers were small. He said the main threat facing U.S. and coalition forces came from supporters of ousted leader Saddam Hussein.
"Foreign fighters are coming in and it is not correct to say that there are floods of foreign fighters coming in or thousands. The number is small," he said.
The "main problem" facing the coalition, he added, is "agents of the former regime."
Iraq Governing Council chairman Jalal Talabani made the request for U.N. recognition of a timetable for Iraqi self-rule in a letter that arrived Monday at the Security Council.
Security Council members have already started discussing a possible new resolution to reflect the Nov. 15 agreement between the U.S.-led Coalition and the Governing Council on an accelerated timetable to restore Iraqi self-government.
But many Security Council members were waiting for Iraq's Governing Council's official response before moving ahead.
Under a proposed general law, Talabani said, a provisional legislative body "that will guarantee a wide representation of all segments of Iraqi society" will be selected by the end of May. This legislative body will elect a provisional Iraqi government no later than the end of June.
Once a provisional government is elected, "the Coalition Provisional Authority will be dissolved and the (U.S.-British) occupation … will end and the role of the Governing Council will end," Talabani said.
The transitional government would organize elections by March 15, 2005 for a constitutional convention, he said. This would include conducting a census, adopting election regulations, a register of voters and regulations for political parties, the press and meetings.
The convention would draft a constitution that would be ratified in a referendum and a new Iraqi government would be elected by Dec. 31, 2005, Talabani said.