WASHINGTON — U.S. Defense officials said Thursday that Iraqi insurgents have sharply curtailed the use of their most powerful roadside bombs, weapons American officials repeatedly have charged are being smuggled into the war zone from Iran.
But Defense Secretary Robert M. Gates said it was too soon to tell whether the decline in the use of the munitions resulted from an Iranian pledge to stem the flow of weaponry between the two countries. Tehran has denied that it is providing munitions to Iraqi insurgents.
The bombs, known as explosively formed penetrators, or EFPs, are a favored weapon of Shiite Muslim militias and can pierce the toughest armor the U.S. Army has fielded in Iraq.
Army Lt. Gen. Raymond T. Odierno, the day-to-day military commander in Iraq, said 53 of the armor-piercing bombs were found on roads in October -- 30 that detonated and 23 that were discovered before they exploded. That is down from 99 in July and 78 in August. In September, 52 exploded or were detected.
The decline in such attacks, Odierno said, coincided with a drop in the number of U.S. military casualties, civilian killings and overall deaths.
"I believe we have achieved some momentum," he said. "Although it is not yet irreversible momentum, this positive momentum has set the conditions for political accommodation, economic development and basic services to progress."
Gates, in a news conference later, refused to say that the developments indicated the U.S. was winning in Iraq.
The Los Angeles Times reported in September that Iraqi Prime Minister Nouri Maliki had secured a pledge from Iranian President Mahmoud Ahmadinejad to help cut off weapons, funding and other support to militants in Iraq.
"It is my understanding that they have provided such assurances," Gates said at the Pentagon, confirming the deal. "I don't know whether to believe them. I'll wait and see."
State Department officials have accused the Iranian government of allowing weapons to be shipped into Iraq. Gates said again Thursday that he had no direct evidence that top Iranian officials have knowledge of smuggling, although he suspects they do.
"My guess is the highest levels are aware," he said.
The military said armor-piercing explosives have been used by the two main Muslim sects, but are most closely associated with Iraqi militias that have ties to Iran, such as elements of radical Shiite cleric Muqtada Sadr's Mahdi Army.
Odierno credited the overall decline in violence in Shiite areas to the U.S. military's success in capturing or killing leaders of militia groups, as well as a cease-fire that Sadr declared in August, halting attacks by his militia against American forces.
U.S. military leaders have discussed the reasons behind the decline in violence by Shiite militias, and a senior Defense official said the Pentagon leadership had yet to determine whether Iran was behind the reduction.
The official said that in addition to possible Iranian moves, the Pentagon has been looking closely at the Mahdi Army for signs of change or a shift in tactics. Some analysts have concluded that the reduction in sectarian violence is a result of the elimination or flight of ethnic and religious minorities from Baghdad neighborhoods. But Odierno, speaking by satellite from Baghdad, said that such moves toward segregation had largely stopped at the beginning of the year.
The U.S. military has reported the discovery of bomb-making factories in Iraq. But American officers have said the most dangerous and largest devices have been smuggled in from Iran.
The military has presented limited evidence of the smuggling, and some critics have questioned whether very many explosives are brought from Iran. A senior Pentagon official said Thursday that providing more detailed evidence could help smugglers elude capture or better disguise weapons.
Odierno said that, in addition to devices found planted on roads, his forces in recent weeks had uncovered "staggering" caches of explosives that had yet to be put in place. Over the last two weeks, he said, U.S. forces have found 136 fully-assembled armor-piercing bombs and materials to make 359 more.
But Odierno said such findings did not mean Tehran was continuing to ship weaponry across the border. He said there was evidence that the recently discovered caches had been delivered in January, before Iran is said to have made its commitment to curtail such shipments.
Growing tension between Washington and Tehran over Iraq, coupled with hostile rhetoric from the Bush administration over Iran's nuclear ambitions, has heightened fears about a possible military clash.
Sen. Jim Webb (D-Va.) sent a letter to the White House, co-signed by 29 senators, condemning recent administration statements and reminding Bush that "no congressional authority exists for unilateral military action against Iran."
But Gates reiterated Thursday that he favors nonmilitary solutions to the Iranian problem.
"Everybody is agreed that the United States' approach to dealing with the Iranian problem now is to focus on economic sanctions and on diplomacy, and I don't think there is any difference within the government on that principle," Gates said.