The top uniformed U.S. military officer told Congress Tuesday that Iran is directly jeopardizing peace in Iraq, prompting fresh calls from senators that the U.S. pursue diplomatic talks with Tehran.

Adm. Michael Mullen, chairman of the Joint Chiefs of Staff, said that "irresponsible actions" by Iran's Revolutionary Guard "directly jeopardize" peace in Iraq.

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"Restraint in our response does not signal lack of resolve or capability to defend ourselves against threats," Mullen told the Senate Appropriations Defense Subcommittee.

Sen. Arlen Specter, a member of the panel, said he is most concerned about the Bush administration's refusal to fully engage Iran in diplomatic discussions. U.S. diplomats have met with Iranian officials as part of a broader regional conference on Iraq, but insisted that discussions be limited to Iraq.

The U.S. accuses Iran of trying to develop a nuclear weapon, supporting terrorist groups and encouraging violence in Iraq by providing weapons and other material aid to Shiite militants there.

Specter, a Republican, said past talks with the Soviet Union, Libya and North Korea proved helpful and that similar progress could be made with Iran.

If the government refuses to engage Iran, "we're missing a great opportunity to avoid a future conflict," Specter said.

Democratic Sens. Patrick Leahy and Dianne Feinstein said they agreed and urged Defense Secretary Robert Gates to challenge President George W. Bush's policy on Iran.

Gates said he supports sitting down with officials from Tehran, but only after the U.S. has developed significant leverage. In such cases as Libya and North Korea, these countries were seeking to relieve economic pressures imposed by sanctions, Gates said.

"The key here is developing leverage, either through economic or diplomatic or military pressures on the Iranian government so they believe they must have talks with the United States because there is something they want from us, and that is the relief of the pressure," Gates said.

Mullen also told the Senate panel that violence in Afghanistan is increasing, as security in Iraq has improved. Accordingly, the U.S. is deploying more troops to Afghanistan and encouraging local forces to do the same to contend with a growing insurgency, increased attacks and a burgeoning drug trade.

"In short, a stable Iraq and Afghanistan that are long-term partners and share our commitment to peace will be critical to achieving regional stability and security," Mullen said.

"This will require years, not months," he added, "and will require the support of the American people, our regional allies and concerted action by the Iraqi and Afghan people and their leaders."

Mullen also reiterated the view of senior military and intelligence officials that the next near-term attack on the U.S. would most likely come al-Qaida forces regrouping in Pakistan along the Afghanistan border. The U.S. reiumburses Pakistan for military strikes against terrorists in the region, but the area remains largely ungoverned.

"It's a very difficult problem because this is sovereign territory" belonging to Pakistan, Mullen said.