An assessment by U.S. spy agencies concludes that Iran is prepared to launch terrorist attacks inside the United States, highlighting new risks as the Obama administration escalates pressure on Tehran to halt its alleged pursuit of an atomic bomb.

In congressional testimony Tuesday, U.S. intelligence officials indicated that Iran has crossed a threshold in its adversarial relationship with the United States. While Iran has long been linked to attacks on American targets overseas, U.S. officials said they see troubling significance in Tehran’s alleged role in a plot to assassinate the Saudi ambassador in Washington last year.

U.S. officials said they have seen no intelligence to indicate that Iran is actively plotting attacks on U.S. soil. But Director of National Intelligence James R. Clapper Jr. said the thwarted plot “shows that some Iranian officials — probably including Supreme Leader Ali Khamenei — have changed their calculus and are now more willing to conduct an attack in the United States in response to real or perceived U.S. actions that threaten the regime.”

The warning about Iran’s more aggressive stance was included in written testimony that Clapper submitted to Congress on Tuesday as part of the intelligence community’s annual assessment of the nation’s most serious security threats.

On other fronts, U.S. intelligence officials said that al-Qaeda has been badly degraded, but they expressed rising concern over

alleged cyber-espionage by China, the leadership transition in

nuclear-armed North Korea and the uncertain prospects for Afghanistan after U.S. forces eventually depart.

Still, Clapper, CIA Director David H. Petraeus and other top U.S. intelligence officials spent much of Tuesday’s hearing before the Senate Intelligence Committee fielding questions about Iran.

Washington is eyeing the country’s nuclear ambitions, long a source of simmering concern, with new urgency. Reports suggest that Iran is closing in on the ability to develop a bomb, and a series of explosions, assassinations and computer attacks targeting the country’s nuclear program have led many outside analysts to conclude that a covert conflict is already underway.

Iran says its nuclear efforts are for peaceful, energy-producing purposes and has blamed the United States and Israel for mysterious developments including the apparently targeted killing of yet another Iranian nuclear scientist in Tehran on Jan. 11, as well as an earlier cyberattack on the country’s largest uranium-enrichment facility.

In his State of the Union address last week, President Obama raised the threat of military intervention to halt Iran’s alleged pursuit of an atomic bomb, saying he would “take no options off the table to achieve that goal.”

Defense Secretary Leon E. Panetta added new rhetorical heat during an interview with CBS’s “60 Minutes” program that aired Sunday, saying that Iran was probably one year away from being able to build a bomb. He added that it could take a year or two beyond that to develop the ability to deliver a warhead by missile.

“That’s a red line for us, and it’s a red line, obviously, for the Israelis,” Panetta said. “If we have to do it, we will do it,” he added, declining to elaborate on what “it” means.

It was against this backdrop that Clapper and others described the efforts by the United States and Israel to move Iran off that nuclear course. Clapper said that escalating economic sanctions have pushed Iran to the brink of a currency crisis, but he acknowledged that the measures have had little or no impact on Iran’s nuclear aims.

“The sanctions as imposed so far have not caused them to change their behavior or their policy,” Clapper said.

He and others testifying Tuesday indicated that their assessment of Iran’s willingness to launch attacks in the United States stems mainly from a more-detailed understanding of the country’s role in the alleged plot to assassinate the Saudi ambassador, Adel al-Jubeir.

As described by U.S. officials in October, the convoluted scheme was to rely on assassins from a Mexican drug cartel to carry out the killing at a restaurant in Washington.

U.S. officials said the plot was devised by an Iranian American with ties to Iran’s Revolutionary Guard Corps. But the plan was foiled when the would-be operative mistakenly hired a paid informant of the Drug Enforcement Administration to carry it out. Iranian officials have denied any role in the plot.

It was “so unusual and amateurish that many initially doubted that Iran was responsible,” Sen. Dianne Feinstein (D-Calif.), chairman of the Senate Intelligence Committee, said in Tuesday’s hearing. “Well, let me state for the record, I have no such a doubt.”

Experts said Iran’s willingness to back such a scheme may reflect a sense among Iran’s leadership that prevailing against the United States and Israel may require adopting new, lower-percentage means of carrying out attacks.

“I see the Iranians feeling that they are under siege,” said Daniel Byman, an Iran expert at Georgetown University and a former CIA analyst. Given Iran’s resources and ties to terrorist groups, including Hezbollah, Byman said that it is “plausible” that Iran already has agents inside the United States.

Clapper’s testimony also called attention to other emerging national security concerns, including cyber-related threats from China and Russia and the diminished but persistent danger to the United States posed by al-Qaeda.

This year’s assessment was the first to evaluate the terrorist network since its founder and leader, Osama bin Laden, was killed in a U.S. commando raid in May. That blow, combined with the toll taken by subsequent strikes and raids, has destroyed al-Qaeda’s core.

As a result, Clapper said in the testimony, the United States is entering a “critical transitional phase for the terrorist threat,” in which smaller-bore strikes from regional nodes are more likely than elaborate, mass-casualty plots.

If the pressure on al-Qaeda can be maintained, “there is a better-than-even chance that decentralization will lead to fragmentation,” Clapper said in his prepared statement. The terrorist group “will seek to execute smaller, simpler plots to demonstrate relevance to the global jihad.”

The group’s affiliate in Yemen continues to be seen as the most likely source of plots targeting the United States. But the death of U.S.-born cleric Anwar al-Awlaki in a CIA drone strike in Yemen last year has at least temporarily eroded the affiliate’s ability to mount international attacks, officials said.