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

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

The start of August has been among the bloodiest stretches for American troops since the 2003 invasion of Iraq, and non-active duty service members have borne the brunt. The 20 Marines killed in two 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.

At a later news conference, Air Force Brig. Gen. Donald Alston, the military spokesman, said, “The numbers that we see indicate that (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 they 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; 2nd Lt. Selena Owens, a spokeswoman with the unit, 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 non-combat-related incident. On Thursday evening, Owens 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 Muqtada al-Sadr. Four were killed and five wounded, according to Iraqi army Lt. Col. Halkot Omar.