WASHINGTON — Even as al-Qaida’s ability to attack the United States continues to diminish, Iran is more willing to attack the United States and American interests overseas, the top U.S. intelligence official told Congress on Tuesday.

Director of National Intelligence James R. Clapper said that a failed 2011 plot to assassinate the Saudi ambassador to the United States in Washington has convinced U.S. intelligence officials that the leaders of the Iranian government are increasingly likely to support attacks on U.S. soil.

“Some Iranian officials — probably including Supreme Leader Ali Khameni — have changed their calculus and are now willing to conduct an attack in the United States,” said Clapper in written testimony submitted to Senate Select Committee on Intelligence in advance of his appearance during a hearing about all security threats facing the United States. Iran continues to move forward with its ability to produce weapons-grade nuclear material, he said.

In October, DEA and FBI agents disrupted a bomb plot to kill Adel Al-Jubeir, the Saudi Arabian ambassador to the United States and possibly more than 100 bystanders in a busy Washington restaurant. The plot, which the United States believes was directed by the Iranian Revolutionary Guard Corps and involved a Mexican drug cartel, never moved beyond the planning stages.

At the same time, the U.S. intelligence officials have determined that al-Qaida’s ability to launch a large-scale attack inside the United States has been degraded by an aggressive campaign of targeted U.S. airstrikes against its senior leadership, and, in the near future, al-Qaida may no longer be the most significant threat facing the country.

President Obama gave a frank defense of the Central Intelligence Agency’s covert use of drones to strike suspected terrorists in Pakistan and elsewhere during a live Web interview with members of the public on Monday. He said that the program had not killed a large number of civilians.

“It is important for everybody to understand that this thing is kept on a very tight leash,” Obama said.

The campaign against al-Qaida has left the organization without a galvanizing central leadership, said Clapper. No charismatic leader has come forward to replace Osama bin Laden since his killing by Navy SEALs last year, and Clapper said that there is a “better-than-even chance” that the disarray among al-Qaida’s central leadership based in Pakistan “will lead to fragmentation of the movement within a few years.”

Intelligence analysts are seeing signs that al-Qaida affiliates in North Africa and Iraq are more focused on regional battles than planning and launching attacks inside the United States. Al-Qaida’s cell in Yemen, al-Qaida in the Arabian Peninsula, despite the killing of the American-born cleric and propagandists Anwar al-Awlaki in September, continues to be the most likely group to launch a successful attack on U.S. soil.

“Lone actors” and “criminals” inspired by terrorist leaders but not part of a terrorist network could conduct “limited attacks” inside the U.S. in the next year, said Clapper. Clapper specifically described as “unlikely” a terrorist attack using a dirty bomb, chemical weapons or deadly germs.

CIA director David H. Petraeus and FBI director Robert Mueller are scheduled to testify with Clapper during the hearing in front of the Senate intelligence committee on Tuesday, as well as four other top U.S. intelligence officials.

