The written statement did not provide any details on what types of attacks Mr. Clapper thought were possible, and senators did not ask him about it during the panel’s annual session to review global threats to the United States.

The session was the first such hearing since the death of Osama bin Laden last May, and Mr. Clapper used the opportunity to say that sustained pressure from the United States and its allies will probably reduce Al Qaeda’s remaining leadership in Pakistan to “largely symbolic importance” over the next two to three years as the terrorist organization fragments into more regionally focused groups and homegrown extremists.

Flanked by senior intelligence officials from throughout the government, Mr. Clapper also noted the rising volatility in the Middle East and North Africa after the popular uprisings of the Arab Spring, increasing threats of cyberattacks against government and private business computer systems, continued tensions with North Korea over its nuclear program and rising drug-fueled violence in Mexico and Central America that threatens to spill over the border.

As American diplomats step up their efforts to broker a peace deal with the Taliban and other militants to end the war in Afghanistan, Mr. Clapper also defended the administration’s discussions of preliminary trust-building measures, including a possible transfer of five Taliban prisoners from Guantánamo Bay, Cuba.

Senator Saxby Chambliss, a Georgia Republican who is the committee’s vice chairman, signaled Congressional opposition to the plan, which administration officials have said would not release the prisoners outright but transfer them to authorities in Qatar, where the Taliban are setting up an office to hold political talks. “We should not transfer these detainees from Guantánamo,” Mr. Chambliss said, citing American intelligence assessments warning of the risks the prisoners posed.

Mr. Clapper acknowledged that the Taliban remained “a resilient, determined adversary” and underscored that any deal involving prisoners would hinge on “where these detainees might go and the conditions in which they would be controlled or surveilled.”

As Taliban leaders debate whether to fight or cut a deal, the death of Bin Laden has severely weakened a Qaeda leadership that was already reeling from the death or capture of several other top leaders. The losses have forced the organization to rely more heavily on affiliates in such places as North Africa, Iraq and Yemen, as well as individual “lone wolf” extremists in the United States.

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Intelligence officials say that continued pressure by the United States and its allies — including drone strikes, efforts to dry up terrorists’ financing and campaigns to counter extremist recruiting propaganda — are likely to fragment this already decentralized movement.

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“As long as we sustain the pressure on it, we judge that core Al Qaeda will be of largely symbolic importance to the global jihadist movement,” Mr. Clapper said in his opening statement.

Of all the affiliates that have sprouted up over the past decade, intelligence analysts say that the Qaeda arm in Yemen, Al Qaeda in the Arabian Peninsula, poses the greatest immediate threat to the United States. Mr. Clapper said that the death last September of Anwar al-Awlaki, an American-born cleric who was a top propagandist and operational planner for the Yemen affiliate, “probably reduces, at least temporarily, A.Q.A.P.’s ability to plan transnational attacks.”

Over all, Al Qaeda has struggled to keep pace with events unfolding as result of the Arab Spring, Mr. Clapper said, warning, however, that “prolonged instability or unmet promises of reform would give Al Qaeda, its affiliates and its allies more time to establish networks, gain support and potentially engage in operations, probably with less scrutiny from local security services.”

The domestic instability in Syria could potentially escalate into regional crises, and American intelligence officials were wary on Tuesday of being pinned down on how long the government of President Bashar al-Assad could survive and what would replace it if it fell.

“It’s a question of time before Assad falls, but that’s the issue; it could be a long time,” Mr. Clapper told senators. “The opposition continues to be fragmented.”

David H. Petraeus, the director of the Central Intelligence Agency, said the opposition had shown increasing resilience in the face of stepped-up attacks by Syrian military forces in suburbs of Aleppo and Damascus. “It has shown, indeed, how substantial the opposition to the regime is and how it is, in fact, growing and how increasing areas are becoming beyond the reach of the regime’s security forces,” he said.

Hopscotching around the world in his remarks, Mr. Clapper singled out Iran for special attention in both his opening comments and a more detailed written statement.

He reiterated the American intelligence assessment that “Iran is keeping open the option to develop nuclear weapons, in part by developing various nuclear capabilities that better position it to produce such weapons, should it choose to do so.”

He added, “We do not know, however, if Iran will eventually decide to build nuclear weapons.” The United States also faces evolving cyberthreats from nations like Russia and China, as well as nonstate entities. Robert S. Mueller III, the director of the Federal Bureau of Investigation, said his agency was beginning to reorganize to combat this. “Down the road, the cyberthreat, which cuts across all programs, will be the number one threat to the country,” he said.

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Nearly a dozen senior administration officials, including the homeland security secretary, Janet Napolitano, are scheduled to brief senators on Wednesday about the latest cyberthreats and American responses.