WASHINGTON - The Iranian government has decided "at the most senior levels" to rein in the violent Shiite militias it supports in Iraq, a move reflected in a sharp decrease in sophisticated roadside bomb attacks over the last several months, according to the State Department's top official on Iraq.

Tehran's decision does not necessarily mean the flow of those weapons from Iran has stopped, but the decline in their use and in overall attacks "has to be attributed to an Iranian policy decision," David Satterfield, Iraq coordinator and senior adviser to Secretary of State Condoleezza Rice, said in an interview.

U.S. Ambassador to Iraq Ryan Crocker said that the decision, "should [Tehran] choose to corroborate it in a direct fashion," would be "a good beginning" for a fourth round of talks between Crocker and his Iranian counterpart in Baghdad.

The Pentagon has been more cautious in describing Iran's role in changes in Iraq. A Defense Department report released Wednesday emphasized that support for militia groups by Tehran's Shiite government remains "a significant impediment to progress." And Defense Secretary Robert Gates said Friday that "the jury is out" on whether Iran is playing a less-destructive role.

One U.S. official, familiar with intelligence reports on Iraq but unwilling to be identified by name or agency, said the conclusion that a high-level policy decision was made in Tehran is "in the strike zone" of intelligence assessments. Iran "would definitely like to maintain some degree of influence over the militias" and other players in Iraq, the official said. Iran remains deeply involved in Iraqi political and economic affairs.

The Bush administration has said that Iran maintains a widespread intelligence network in Iraq, with blurred lines between political operatives and those with direct involvement in militia violence. Rather than lessening its influence in Iraq, the official said, Iran has opted for "a creative shift in tactics" as violent militia action - some of it directed against Shiites - has turned many Iraqis against them.

Satterfield agreed that Iran was not acting out of "altruism," but rather from "alarm at what was being done by the groups they were backing in terms of their own long-term interests."

At a news conference Friday, Rice sidestepped an opportunity to criticize Iran. The United States, she said, remains "open to better relations" with Iran, adding: "We don't have permanent enemies."

The administration has long been reluctant to ascribe anything but malevolent motives to Tehran. Although a National Intelligence Estimate released this month concluded that Iran stopped its nuclear weapons program four years ago, the White House has emphasized its conclusion that the program, which Iran has consistently denied, actually existed.

The administration is far from declaring a fundamental change in Iran's attitude toward and objectives in Iraq, or in judging that the new direction is permanent.

But "we have seen such a consistent and sustained diminution in certain kinds of violence by certain kinds of folks that we can't explain it solely" by internal factors in Iraq, Satterfield said.

He declined to discuss specific evidence. "We are confident that decisions involving the strategy pursued by the IRGC are made at the most senior levels of the Iranian government," Satterfield said, referring to the Iranian Revolutionary Guard Corps. The administration has used that formulation in the past to insist that IRGC training and supplies for militias in Iraq were ordered by Tehran's highest clerical leaders.