BAGHDAD -- Iran's commitments to stem the flow of weapons and explosives into Iraq "appear to be holding up" and have contributed to a sharp drop in roadside bombs across the country, a U.S. general said Thursday.

Maj. Gen. James Simmons, a deputy corps commander, said that in October, U.S. forces logged 1,560 cases in which bombs were either found and exploded.

That compared with 3,239 incidents in March, he said. The October figure was the lowest since September 2005, he added.

In August, Iraqi authorities said Iranian officials promised Iraqi Prime Minister Nouri al-Maliki that they would stem the flow of weapons and ammunition smuggled to extremists in Iraq.

Since then, U.S. military officials have reported finding fewer "explosively formed penetrators," a particularly deadly form of roadside bomb they believe come from Iran.

"We believe that the commitments that the Iranians have made appear to be holding up," Simmons said.

Iranian officials have publicly denied smuggling weapons to Shiite extremists. But U.S. authorities insist penetrator bombs are the signature weapon of Shiite militants.

Simmons said that penetrator bombs were still being found in Iraq but they appeared to have entered the country months ago.

U.S. authorities said penetrators were used in an attack Wednesday against a U.S. Stryker vehicle near an entrance to the Green Zone, killing an American soldier and wounding five others. Iraqi police said two Iraqi civilians also were killed.

It was the first major attack against a U.S. military vehicle in that area in the last four or five months, Simmons said.

He said the vehicle was struck by "an array" of penetrators. The attack occurred in one of the most heavily protected areas of the capital, raising questions how the explosives could have been planted without collusion from Iraqi police or soldiers.

The general said U.S. and Iraqi authorities were investigating the attack.

Simmons said U.S. authorities also were encouraged by an increase in tips from Iraqi citizens about weapons caches, which he interpreted as a sign the public was turning against both Shiite and Sunni extremists.

"We had found more caches by May of this year than in all of 2006," he said.

Simmons said most of the roadside bomb attacks recently had occurred in Sunni areas north of Baghdad.

Northern Iraq also has seen a spike in violence in recent months as extremists were pushed from strongholds in and around Baghdad.

In Kirkuk, an ethnically mixed city 180 miles north of Baghdad, a suicide bomber rammed his car into a police patrol in Kirkuk, killing six people and wounding more than 20, said police Brig. Sarhad Qadir.

The city has seen a rise in bloodshed ahead of a planned census and referendum to determine the future of the city — whether it will join the semiautonomous Kurdish region on its border, or remain under Baghdad's control.

The bomber's apparent target was the six-car convoy of a senior Kurdish police officer, Brig. Gen. Khattab Omar, who heads the city police department's quick response force, Qadir said.

Three of Omar's officers were killed, along with three civilians, but the commander survived with serious injuries to his chest and head, Qadir said. Omar was being evacuated to a larger hospital, he said.

Video from AP Television News showed a charred Iraqi Humvee being towed from the scene.

Many of the 21 people wounded were children who had been walking to school when the bomber struck. APTN video from inside a nearby hospital showed a young girl in a school uniform, drenched in blood. A child's shoe could be seen peeking out from under a tarp covering corpses — suggesting at least one of the dead civilians was a child.

Also Thursday, the U.S. military said a U.S. soldier had been killed a day earlier in an explosion in Diyala province that wounded four other soldiers.

At least 3,865 members of the U.S. military have died since the beginning of the Iraq war in March 2003, according to an AP count.

Meanwhile, Iraqi officials said they were investigating whether U.S. troops had mistakenly killed about two dozen anti-al-Qaida fighters earlier this week north of Baghdad.