A senior U.S. military officer said Wednesday that much of the preparation for the wave of bombings that struck Baghdad on Wednesday originated in the Euphrates River valley west of the Iraqi capital, an area that officials say is used by insurgent leader Abu Musab Zarqawi as his base of operations.

The officer, assigned to the U.S. Central Command, said there was "strong evidence" linking the bombings to Zarqawi, beyond an assertion of responsibility posted on the Internet. While declining to elaborate on the evidence, the officer indicated that U.S. authorities already have some idea of the planning and organization behind the attacks.

"We believe strongly he doesn't have a foothold in Baghdad," said the officer, who spoke on condition of anonymity. "He imports these attacks from his base in the Euphrates River valley."

The valley region has been the focus of an intensified hunt for Zarqawi for months. Officials have said that Zarqawi was nearly captured there in February by a U.S. Special Operations task force devoted full time to searching for him.

Arrests of a number of fighters described as lieutenants in Zarqawi's network have led to statements by U.S. military officials that the network had been significantly degraded. A recent decrease in car bombings and other attacks in Baghdad attributed to Zarqawi also had led to speculation about a weakening of his ranks.

But the shocking scope of Wednesday's attacks indicated Zarqawi is still capable of tremendous violence. It also suggested he might have adjusted his tactics, reducing the frequency of attacks in the interest of having a greater impact when he does strike.

"Just when you think you have your foot on his throat, he can come back," the officer said in an interview. "He marshals his resources in order to have days like today to get himself into the news."

The Baghdad attacks appeared as well to call into question a notion that top U.S. military officers in the region had begun to articulate about insurgent activity shifting out of the cities and moving west.

In interviews this week before the Baghdad bombings, several senior Central Command officers said that U.S. and Iraqi military action earlier this year in central and northern Iraq had ended up pushing much insurgent activity out of large urban areas and into the sparsely populated western reaches of the country.

They noted that counterinsurgency operations had shifted as well. Once centered in such major cities as Baghdad, Mosul and Tikrit, the operations have moved west up the Euphrates valley and into smaller towns closer to Iraq's border with Syria.

This trend, the officers said, represented a setback for the insurgents, who would prefer to keep the fight in Iraq's main population centers for maximum effect. "The insurgency is much more pushed to the west in Iraq this year than it was in the previous years, and I actually regard that as a sign that the insurgency is having a hard time getting established elsewhere," Army Gen. John P. Abizaid, the head of Central Command, said before the report of the bombings.

At the same time, Abizaid cautioned that progress in Iraq "won't be a straight line" and that the country "is going to be a pretty difficult security environment for a while."

Abizaid was unavailable for comment after the scale of Wednesday's bombings became evident. But the other Central Command officer said the assault did not change the view that the centers of operations for Zarqawi followers and for the Sunni Muslim fighters who dominate Iraq's insurgency were now west of the capital.

Another officer, who also discussed the shift on condition of anonymity, said there had been no deliberate U.S. military strategy to push the insurgents west. Rather, he said, the movement was a consequence of an earlier strategy focused on securing large urban areas.

This development would appear to pose new challenges for U.S. forces, which have tended to be spread thinner in the west than in central and northern Iraq. Indeed, some members of the U.S. military in the western province of Anbar, a largely barren expanse, have been quoted in news accounts this year appealing for more troops.

The U.S. military has more than 32,000 troops in Anbar, 22,000 of them Marines. Abizaid, in an interview, dismissed the suggestion that more were needed, noting that increasing numbers of Iraqi forces were conducting operations in the area. He also expressed confidence in the judgments of commanders to apportion their forces.

"I believe the commanders clearly know where they have to move their forces in order to have the effect they want to have," he said.

"The insurgency is much more pushed to the west," Gen. John P. Abizaid of the U.S. Central Command said before the Baghdad bombings.