(Reuters) - The possible release of detained Taliban leaders is likely to join Iran's nuclear ambitions at the top of a busy agenda when the top seven American intelligence chiefs testify before the Congress this week.

Director of National Intelligence James Clapper -- as well as the heads of the CIA, FBI, Defense Intelligence Agency, National Counterterrorism Center and State and Homeland Security department intelligence units -- will be grilled on worldwide threats at a pair of open hearings.

The hearings -- before the Senate Intelligence Committee on Tuesday and its House of Representatives counterpart on Thursday -- are an annual ritual. But they offer a rare opportunity for legislators publicly to raise sensitive national security topics that are usually discussed only in secret briefings.

Lawmakers have expressed concern about the Obama administration's efforts to engage in peace talks with the Afghan Taliban, including consideration of the release of five Taliban leaders incarcerated at Guantanamo as a confidence building gesture, sources close to the committee said.

Sen. Saxby Chambliss, R-Ga., vice chairman of the Senate intelligence committee, told Reuters at the World Economic Forum in Davos last week that he and the panel's Democratic chairwoman, Dianne Feinstein of California, had twice written to the Obama administration raising strong objections to the proposed move, which purportedly would involve the release of the Taliban prisoners into Afghan government custody.

Chambliss said he and Feinstein first wrote to Secretary of State Hillary Clinton. We've gotten a response from Clinton, he said, declining to discuss the classified contents of her letter.

But he hinted that Clinton's response did not mollify them, adding that he and Feinstein then wrote a second letter to President Barack Obama.

Chambliss said there was every reason to believe some of the five Taliban detainees were involved in the death of CIA officer Johnny Micheal Spann during an uprising by Taliban prisoners at a fortress outside the Afghan city of Mazar-i-Sharif in November 2001.

I think it's bad policy. We don't negotiate with terrorists. We never have, Chambliss said, calling the detainees five of the meanest, nastiest killers in the world.

Two detainees slated for possible release, former senior Taliban army commanders Mohammed Fazl and Noorullah Noori, were held at the historic Qala-i-Jangi fortress outside Mazar-i-Sharif when the prison revolt erupted in 2001.

But a senior administration official, speaking on condition of anonymity, insisted earlier this month that he knew of no evidence that they were involved in the death of Spann, who was surrounded and killed by rioting prisoners.

CONGRESS NOT NOTIFIED

Chambliss said Obama had not yet notified Congress of the administration's plan to transfer the prisoners from Guantanamo, a step which is required by a law which the president signed on Dec. 31.

It's the president's call. There's an ongoing conversation about it, let me say that, said Chambliss. To date, all discussions the Administration has had with Congress on the prisoner transfer issue have taken place behind closed doors.

Iran's nuclear progress and threats to Persian Gulf shipping and U.S. interests are almost certain to be key topics of discussion, with U.S. intelligence officials expected to offer a less alarming take on Iranian activity than those voiced by some conservative and Israeli analysts.

U.S. officials have told Reuters they believe that Iran's leaders have not yet made a decision to build nuclear weapons, an assessment which the U.S. intelligence community has held since it published extracts of a controversial National Intelligence Estimate on the issue in 2007.

U.S. officials say the reason Iranian leaders have not made a decision to build a bomb is because they are still weighing the costs and benefits of doing so.

But while a lot of what Iran is doing in its nuclear program has civilian applications, U.S. officials also believe that Tehran is keeping its options open regarding the building of a bomb -- adding to the air of ambiguity surrounding Iran's intentions and dealings with the U.S. and the outside world.

Other subjects which officials said might come up during this week's hearings are the stability of North Korea's regime and the future of its nuclear program after the death last month of Kim Jong-il and his replacement by his son Kim Jong-un; the recent discovery by authorities in Thailand of possible stockpiling of bomb-making materials by an individual with alleged links to the Lebanese militant group Hezbollah; and drug-related violence and instability in Mexico.

The activities of homegrown U.S. militants could also be raised at the hearings. The FBI and other domestic law enforcement agencies have been particularly concerned about lone wolf militants who become radicalized via the internet and plot violence without catching the attention of authorities.

We have to constantly be vigilant against a range of threats. Terrorism didn't begin with (Osama) bin Laden, it's not over with his death. There are other al-Qaida, al-Qaida-related groups, and we have the growth of homegrown extremists, Homeland Security Secretary Janet Napolitano said on Monday.

Last week in Baltimore, a man pleaded guilty to attempting to bomb a military recruiting center. Investigators said he had no connection to international militants but had been trying unsuccessfully to recruit others to join his plot until he was trapped by authorities in a sting operation.

Earlier in January, a home-grown militant in Tampa was arrested in connection with a plot to car-bomb nightclubs and then attack crowds of onlookers with guns or a suicide vest.

(Reporting By Mark Hosenball and Warren Strobel; additional reporting by Tabassum Zakaria)

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