Congressmen from both parties expressed fear that time is running out in the fight to stop Tehran from building nuclear weapons.
Syria is widely considered Iran's closest ally in the Middle East. Leaders in both countries have been criticized for brutally repressing democratic reform movements at home while supporting violent Islamic extremism abroad.
"The Iranian and Syrian governments continue their current behavior at ever-increasing risk of isolation," said Wendy Sherman, under secretary of state for political affairs. "Our actions to counter their domestic, regional, and international belligerency are unmistakably escalating the cost of doing business as usual for both countries."
The United States and its allies are "making progress in both Iran and Syria," insisted David Cohen, Treasury under secretary for terrorism and financial intelligence. Despite the recent imposition of a host of new economic sanctions, all options "are on the table" and "we will continue (to develop) new and innovative ways" to punish the two governments if they fail to change, he said.
Sherman and Cohen both issued strong condemnations of the alleged assassination plot.
"This plot was a flagrant violation of international law and a (dangerous) escalation by Iran," Sherman said. "The regime must be held accountable for its actions."
Manssor Arbabsiar, a 56-year-old naturalized U.S. citizen, and Gholam Shakuri, an Iran-based member of Iran's Islamic Revolutionary Guard Corps, have been accused of conspiracy to murder Adel Al-Jubeir, the Saudi ambassador to the United States.
The "murder for hire" plot is a "wake-up call," argued Florida GOP Rep. Ileana Ros-Lehtinen, the chairwoman of the committee. "Imagine how much more blatant (Iran's) aggression would be if it had nuclear weapons."
California Rep. Howard Berman, the committee's top Democrat, said he was "sickened" by the "twisted" plot.
Democrats and Republicans split sharply, however, in their assessment of the administration's handling of Iran and Syria so far. Republicans were particularly critical of the administration's attempt to coordinate international responses through institutions such as the United Nations, long an object of derision among conservatives.
It is "foolhardy and dangerous" to try to work with countries such as Russia and China at the U.N. Security Council, said Ros-Lehtinen, noting that those two countries blocked a resolution earlier this month condemning Syria for its treatment of democratic reform activists.
"Instead of begging for help, we need a realistic policy," she said.
The overall U.S. policy regarding Iran remains largely unchanged, insisted Rep. Steve Chabot, R-Ohio. "It is long past time to jettison this dangerous fantasy" of trying to reach an accommodation with Tehran.
Several Republicans criticized the White House for failing to explicitly endorse regime change in Iran, a step that has been taken with regard to Syria.
"The little fellow from the desert -- (Iranian President Mahmoud) Ahmadinejad -- has to be replaced by his own people," declared Rep. Ted Poe, R-Texas.
Berman defended the administration's response, arguing that significant strides have been made in ways that don't always attract media headlines.
"There is no administration that has spent more time (and had) more focus, and been more effective" in assembling a meaningful international response to regimes like Iran, he asserted.
"No administration has been as forthright and forceful," added Rep. Gerry Connolly, D-Virginia. If progress hasn't been sufficient, "it's not because of a want of trying."
CNN's Alan Silverleib contributed to this report
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