The U.S. military said Thursday that four more GIs were killed Wednesday, a Marine in the central city of Ramadi and three soldiers in a car bombing in southwest Baghdad.

Their deaths, along with those of 14 Marines in western Iraq announced earlier, brought Wednesday's U.S. toll to 18.

The start of August has been among the bloodiest stretches for American forces since the 2003 invasion of Iraq, and non-active-duty service members have been hit hardest. Twenty-one Marines killed in three separate incidents this week were reservists, and the soldiers killed Wednesday were from the Georgia National Guard.

About 30 percent of the 138,000 U.S. troops in Iraq are from the National Guard and reserves.

The news of more U.S. fatalities came on a day in which the Iraqi prime minister announced a new security plan and said Iraqi forces were making the country safer, while the top U.S. military spokesman in Iraq told reporters that data showed the tempo of insurgent attacks was decreasing.

"The security situation is improving, especially Iraqi security forces with regard to both quality and quantity," said Prime Minister Ibrahim Jafari, who outlined a 12-point security program for Iraq on Thursday afternoon.

Under the new plan, four separate intelligence services will be consolidated into a central operation, and responsibility for security of the country's infrastructure -- such as oil pipelines and the power grid -- will be put in the hands of Deputy Prime Minister Ahmed Chalabi. Jafari provided few other details, beyond general pledges to do things such as secure Iraq's borders and improve relations with its neighbors.

At a news conference later, Air Force Brig. Gen. Donald Alston, the military spokesman, said the insurgents "can't generate the same tempo" of attacks as in previous months. The 13 car bombs detonated across Iraq last week represented the fewest since April, Alston said, although he declined to provide data for other forms of insurgent attacks such as roadside bombs.

"This is not an expanding insurgency," Alston added. "What we are seeing is probably the opposite."

Insurgent tactics have gotten more effective and sophisticated, Alston acknowledged, saying the rebels have modified their attacks to counteract improvements in the armor applied to U.S. vehicles, using larger and more lethal forms of explosives. "What we are seeing is an adaptive enemy," he said.

In Baghdad, the three soldiers killed Wednesday night were from the 48th Brigade of the Georgia National Guard, which arrived in Iraq in May. A spokeswoman with the unit, 2nd Lt. Selena Owens, said another soldier was seriously wounded.

The 48th Brigade has lost 11 members in insurgent attacks in less than two weeks and another in a noncombat incident. Thursday evening, she said, soldiers held a memorial service for four soldiers killed by a roadside bomb Saturday.

Elsewhere in Iraq on Thursday, three policemen were killed in the northern city of Kirkuk, and 20 miles to the south, in Daquq, a suicide car bomb detonated near a convoy of vehicles bearing followers of Shiite Muslim cleric Moqtada Sadr. Four were killed and five wounded, according to Iraqi army Lt. Col. Halkot Omar.

Special correspondents Marwan Ani in Kirkuk and Omar Fekeiki in Baghdad contributed to this report.